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: Ie99401a2102c352ffb1193a57aa66a5b96c184aa
Reviewed-on: https://chromium-review.googlesource.com/540556
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>