WebRtcCaptureFromElementBrowserTest.VerifyCanvasCaptureWebGLFrames: enable.

While reproducing the flaky test on Android it was discovered that
randomly commenting at least two of the sequential tests in
canvas_capture_color.html makes the test pass. This change re-enables
the test by splitting it in two pieces, opaque and alpha cases.

Fixed: chromium:1334876
Change-Id: Ifa251fddd34c03c2d7479a7de08747c2f652709d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3822944
Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Markus Handell <handellm@google.com>
Cr-Commit-Position: refs/heads/main@{#1033449}