Improve development workflow with built libchromiumcontent

- Add `--debug_libchromiumcontent` to build libchromiumcontent for debugging
  (shared library build).
- By default, only invoke `gclient sync` the first time to checkout chromium
  source tree. Add `--force_update_libchromiumcontent` switch to force updating.
- Document new options.

The goal is to allow faster edit/compile cycles when debugging/making changes
to libchromiumcontent.

docs/development/chromium-development.md

@@ -12,3 +12,64 @@
 - [Discussion Groups](http://www.chromium.org/developers/discussion-groups)
 
 See also [V8 Development](v8-development.md)
+
+# Chromium development with Electron
+
+It is possible to debug Chromium with Electron by passing
+`--build_debug_libcc` to the bootstrap script:
+
+    $ ./script/bootstrap.py -d --build_debug_libcc
+
+This will download and build libchromiumcontent locally, similarly to the
+`--build_release_libcc`, but it will create a shared library build of
+libchromiumcontent and won't strip any symbols, making it ideal for debugging.
+
+When built like this, you can make changes to files in
+`vendor/libchromiumcontent/src` and rebuild quickly with:
+
+    $ ./script/build.py -c D --libcc
+
+When developing on linux with gdb, it is recommended to add a gdb index to speed
+up loading symbols. This doesn't need to be executed on every build, but it is
+recommended to do it at least once to index most shared libraries: 
+
+    $ ./vendor/libchromiumcontent/src/build/gdb-add-index ./out/D/electron
+
+Building libchromiumcontent requires a powerful machine and takes a long time
+(though incremental rebuilding the shared library component is fast). With an
+8-core/16-thread Ryzen 1700 CPU clocked at 3ghz, fast SSD and 32GB of RAM, it
+should take about 40 minutes. It is not recommended to build with less than 16GB
+of RAM.
+
+## Chromium git cache
+
+`depot_tools` has an undocumented option that allows the developer to set a
+global cache for all git objects of Chromium + dependencies. This option uses
+`git clone --shared` to save bandwidth/space on multiple clones of the same
+repositories.
+
+On electron/libchromiumcontent, this option is exposed through the
+`LIBCHROMIUMCONTENT_GIT_CACHE` environment variable. If you intend to have
+several libchromiumcontent build trees on the same machine(to work on different
+branches for example), it is recommended to set the variable to speed up the
+download of Chromium source. For example:
+
+    $ mkdir ~/.chromium-git-cache
+    $ LIBCHROMIUMCONTENT_GIT_CACHE=~/.chromium-git-cache ./script/bootstrap.py -d --build_debug_libcc
+
+If the bootstrap script is interrupted while using the git cache, it will leave
+the cache locked. To remove the lock, delete the files ending in `.lock`:
+
+    $ find ~/.chromium-git-cache/ -type f -name '*.lock' -delete
+
+It is possible to share this directory with other machines by exporting it as
+SMB share on linux, but only one process/machine can be using the cache at a
+time. The locks created by git-cache script will try to prevent this, but it may
+not work perfectly in a network.
+
+On Windows, SMBv2 has a directory cache that will cause problems with the git
+cache script, so it is necessary to disable it by setting the registry key
+
+    HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Lanmanworkstation\Parameters\DirectoryCacheLifetime
+
+to 0. More information: https://stackoverflow.com/a/9935126