(Re-land after fix to avoid deleting nonexistent textures.)

Blitting from a multisampled RGB8 renderbuffer to an alpha:false WebGL
back buffer is supposed to work, and from an RGBA8 renderbuffer is
supposed to not work. The emulation of IOSurface-backed RGB textures
using RGBA textures didn't handle this case.

Add BindTexImage2DWithInternalformatCHROMIUM to the command buffer to
allow internal format to be overridden; currently this is only
supported on macOS. Use this to bind a second texture to the IOSurface
and use it only for BlitFramebuffer calls from WebGL, fixing up the
alpha channel afterward.

Covered by new conformance test in
https://github.com/KhronosGroup/WebGL/pull/2377 .

BUG=699566
TBR=zmo@chromium.org,ccameron@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2838743002
Cr-Commit-Position: refs/heads/master@{#466878}