Only enable ANGLE_multiview extension on request

The ANGLE_multiview extension was erroneously enabled by default if
available in the GPU passthrough command buffer. This was visible as
the GL_OVR_multiview preprocessor macro being defined in shaders even
if the WEBGL_multiview extension was not enabled.

This is now fixed so that the extension is only enabled when
requested.

BUG=864524
TEST=WebGL conformance tests

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ie6e684fb91af4bb50aac7027ee9f85cd4420fe37
Reviewed-on: https://chromium-review.googlesource.com/1158569Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Cr-Commit-Position: refs/heads/master@{#580107}

gpu/command_buffer/service/gles2_cmd_decoder_passthrough.cc

@@ -661,7 +661,6 @@ gpu::ContextResult GLES2DecoderPassthroughImpl::Initialize(
           "GL_ANGLE_framebuffer_blit",
           "GL_ANGLE_framebuffer_multisample",
           "GL_ANGLE_instanced_arrays",
-          "GL_ANGLE_multiview",
           "GL_ANGLE_pack_reverse_row_order",
           "GL_ANGLE_texture_compression_dxt3",
           "GL_ANGLE_texture_compression_dxt5",