Clip FramebufferGL::readPixels to framebuffer. 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>