Fix sandboxed crashReporter for windows.

- Use `path` module from browser process in sandboxed renderer. This is required
  because the return value of `path.join` is platform-specific, and this is an
  assumtion of crash-reporter.js which is shared between sandboxed and
  non-sandboxed renderers.
- Set `process.platform` and `process.execPath` in sandboxed renderer
  environment. This is required to spawn the windows crash service from
  sandboxed renderer.
- Use a single temporary directory for all crashReporter tests. This is required
  to make tests more deterministic across platforms(since mac's crashpad doesn't
  support changing the crash dump directory). Also make a few improvements/fixes
  to the `uploadToServer` test.

lib/sandboxed_renderer/api/exports/path.js

@@ -0,0 +1 @@
+module.exports = require('electron').remote.require('path')

lib/sandboxed_renderer/init.js

@@ -27,6 +27,7 @@ const preloadModules = new Map([
   ['electron', electron],
   ['fs', fs],
   ['os', require('os')],
+  ['path', require('path')],
   ['url', require('url')],
   ['timers', require('timers')]
 ])
@@ -36,8 +37,9 @@ const preloadSrc = fs.readFileSync(preloadPath).toString()
 // Pass different process object to the preload script(which should not have
 // access to things like `process.atomBinding`).
 const preloadProcess = new events.EventEmitter()
-preloadProcess.platform = electron.remote.process.platform
 preloadProcess.crash = () => binding.crash()
+process.platform = preloadProcess.platform = electron.remote.process.platform
+process.execPath = preloadProcess.execPath = electron.remote.process.execPath
 process.on('exit', () => preloadProcess.emit('exit'))
 
 // This is the `require` function that will be visible to the preload script