Update PixelRef GenerationID when copying data from WebGL to its pixels

This patch is preventing an upcoming Skia change to break the following test:
fast/canvas/webgl/canvas-test.html

Because the GenerationID of the GrPixelRef was not updated, it was causing a false cache hit in SkBitmapCache and thus, was making readPixels return the wrong data.

BUG=413900

Review URL: https://codereview.chromium.org/570703002

git-svn-id: svn://svn.chromium.org/blink/trunk@182105 bbb929c8-8fbe-4397-9dbb-9b2b20218538