Make OffscreenCanvas 2D context support globalAlpha
We have already supported globalCompositeOperation, but globalAlpha is
not exposed yet.
This CL expose this attributes. Similar to the globalCompositeOperation,
the implementation is already there, we just need to expose it in the
.idl file.
The layout test is copied from fast/canvas/2d.composite.globalAlpha.fillPath.html,
with very small tweak.
BUG=659260
Review-Url: https://codereview.chromium.org/2526693003
Cr-Commit-Position: refs/heads/master@{#435634}
8 files changed
