Canvas element that use the alpha attribute don't display correctly.

Current blink is delaying the creation of the backing store until drawing something.
When alpha is false, the backing store must be initialized to opaque black instead of transparent black.

BUG=352018

Review URL: https://codereview.chromium.org/200003002

git-svn-id: svn://svn.chromium.org/blink/trunk@171408 bbb929c8-8fbe-4397-9dbb-9b2b20218538