Fixed bookkeeping bug in management of dimensions of mailbox's texture.

This fixes several problems with rapid resizing of WebGL-rendered canvases.

Also added a WebGL conformance test for this bug.

BUG=299371
TEST=DrawingBufferTest.verifyResizingProperlyAffectsMailboxes

Review URL: https://codereview.chromium.org/105123003

git-svn-id: svn://svn.chromium.org/blink/trunk@163509 bbb929c8-8fbe-4397-9dbb-9b2b20218538