WebGL: optimize to draw WebGL on external destination. e.g. 2D Canvas.

DrawingBuffer::paintCompositedResultsToCanvas()/copyToPlatformTexture() reuses
the mailbox which was created for the compositor, instead of creating a temporary mailbox.
It reduces mailbox creation as well as avoids context switching and glFlush.

BUG=331181

Review URL: https://codereview.chromium.org/665073002

git-svn-id: svn://svn.chromium.org/blink/trunk@184774 bbb929c8-8fbe-4397-9dbb-9b2b20218538