Add alpha channel stride on CanvasCaptureHandler

      This CL adds stride support for cases where canvas has aligned
alpha channel. Note that, this code is still temporary and will be
replaced by libyuv::ARGBToA when available.

BUG= 524218
TEST= http://software.hixie.ch/utilities/js/live-dom-viewer/?saved=3905

Review URL: https://codereview.chromium.org/1826923002

Cr-Commit-Position: refs/heads/master@{#383017}