GitLab

1. Added ContextProviderMain() that instantiates ContentProviderImpl.
2. Updated WebRunnerMainDelegate to run WebRunnerBrowserMain() only
   when process is started with --type=web-context
3. Updated WebRunnerBrowserMainParts to instantiate ContextImpl.
4. Updated ContextProviderImpl to pass --type=web-context to the
   web context process.

Bug: 852145
Change-Id: I9740162ad9006df5e254e6ca3d4bed24640c94ce
Reviewed-on: https://chromium-review.googlesource.com/1134585
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575378}