Prevent 2D canvas flickering after graphics context loss. Drop the references to active mailboxes in Canvas2DLayerBridge when the graphics context is lost.

BUG=265552

Review URL: https://chromiumcodereview.appspot.com/21096003

git-svn-id: svn://svn.chromium.org/blink/trunk@155103 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Clean up callers of shadow() that really just want youngestShadowRoot()

Several callers of Element::shadow() really just want to call Node::youngestShadowRoot() which
makes the code simpler.

Review URL: https://chromiumcodereview.appspot.com/21107003

git-svn-id: svn://svn.chromium.org/blink/trunk@155104 bbb929c8-8fbe-4397-9dbb-9b2b20218538