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This change adds RenderThread::GetShutdownEvent() to the public content API.
This is required for code outside of content/ that wants to safely use
interfaces like SyncChannel.

BUG=

Review URL: https://codereview.chromium.org/186973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255275 0039d316-1c4b-4281-b951-d872f2087c98