In GL, ReadPixels() is allowed to modify memory that corresponds to
pixels outside the framebuffer.
In WebGL it must not do that, so clip the read area to the framebuffer.

Enable corresponding test.

BUG=angleproject:1815

Change-Id: I8113ae417dee7834e63498aec8291ce711bd7513
Reviewed-on: https://chromium-review.googlesource.com/536434
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>