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.

spec/fixtures/api/crash.html

@@ -17,7 +17,7 @@ crashReporter.start({
   }
 });
 if (!uploadToServer) {
-  ipcRenderer.sendSync('set-crash-directory', crashReporter.getCrashesDirectory())
+  ipcRenderer.sendSync('list-existing-dumps')
 }
 setImmediate(function() { process.crash(); });
 </script>