Remove InitializeOneOff from SynchronousCompositor

After r248557, InitializeOneOff can no longer be called multiple times.
Also on Android, it is guaranteed called early in
BrowserMainLoop::BrowserThreadsStarted. So no need for additional calls
every time trying to create context in synchronous compositor.

This was missed because this code path is not exercised by any tests in
chromium.

BUG=
TBR=danakj
NOTRY=true

Review URL: https://codereview.chromium.org/140513009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249099 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249099