Fixes a validation error and rendering artifact in the WebGL test:
textures/misc/tex-image-and-sub-image-2d-with-array-buffer-view

Only applies to masked clears when combined with draw calls using
uniforms or textures.

Also make it easier to inspect layer messages in the debugger.

Bug: angleproject:2912
Change-Id: I4561895439221581b9dbc341d6de5d5a6c2096aa
Reviewed-on: https://chromium-review.googlesource.com/c/1462056
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>