Adjust DrawingBuffer's visibility tracking behavior with lost context

This patch adjusts DrawingBuffer's behavior when a context is lost in
two ways:

1. Always tell DrawingBuffer whether it is visible or not regardless of
   whether the WebGL context is lost. This is needed because visibility
   notifications may arrive from the browser while the context is lost
   and the compositor may also attempt to composite the DrawingBuffer
   during this time.

2. Don't check the current visibility in
   DrawingBuffer::prepareMailbox() if the DrawingBuffer is being
   destroyed. This is because the DrawingBuffer may become hidden right
   before the context is forcibly lost (using WEBGL_lose_context), and
   after that point WebGLRenderingContextBase no longer has a reference
   to the DrawingBuffer to update its visibility state.

BUG=411372
TEST=Added in https://codereview.chromium.org/560023002

Review URL: https://codereview.chromium.org/556303002

git-svn-id: svn://svn.chromium.org/blink/trunk@181808 bbb929c8-8fbe-4397-9dbb-9b2b20218538