WebGL: Insert a sync point on DrawingBuffer::prepareMailbox()

This CL is micro optimization and makes code consistent with Canvas2DLayerBridge::prepareMailbox().

Sync points are needed for only Ubercompositor because they are needed for only
inter-process synchronization.

This change shouldn't be strictly necessary, in that the renderer compositor
doesn't need it (since it uses the same channel), and it will insert a sync point
before sending to the parent.

Nevertheless, this change has two benefits:
1. micro optimization; This change provides a sync point on commit time, so the parent's
context may be able to wait "less". Otherwise, the renderer compositor provides
a sync point on drawing time.
2. Make WebGL code consistent with Accelerated 2D Canvas code.

BUG=332401

Review URL: https://codereview.chromium.org/105813014

git-svn-id: svn://svn.chromium.org/blink/trunk@165326 bbb929c8-8fbe-4397-9dbb-9b2b20218538