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First ensure that main thread offscreen context is created after
on-screen context by keeping count of the number of compositors
that are initialized for hardware draw. Also release the compositor
thread offscreen context when the last compositor releases hardware
draw. Freeing the main thread context is still a TODO.

In software mode, still create StreamTextureFactory, but skip creating
StreamTextureProxy. WebMediaPlayerAndroid already works in this
mode, and it just means in-line video does not work in this case.
But video playback is still successful and user is able to enter fullscreen
to get working playback.

BUG=285349

Review URL: https://chromiumcodereview.appspot.com/23618031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221843 0039d316-1c4b-4281-b951-d872f2087c98