Initialize color_buffer_half_float_rgba in passthrough

If HDR is requested, DCHECK that RGBA_16F is supported.

R=geofflang
TBR=zmo

Bug: 784536
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
Change-Id: Ib775c15c8a0a0d4f89c33f408c3dede2907d49ce
Reviewed-on: https://chromium-review.googlesource.com/779865Reviewed-by: ccameron <ccameron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518407}

components/viz/service/display/gl_renderer.cc

@@ -447,8 +447,10 @@ bool GLRenderer::CanPartialSwap() {
 }
 
 ResourceFormat GLRenderer::BackbufferFormat() const {
-  if (current_frame()->current_render_pass->color_space.IsHDR() &&
-      resource_provider_->IsRenderBufferFormatSupported(RGBA_F16)) {
+  if (current_frame()->current_render_pass->color_space.IsHDR()) {
+    // If a platform does not support half-float renderbuffers then it should
+    // not should request HDR rendering.
+    DCHECK(resource_provider_->IsRenderBufferFormatSupported(RGBA_F16));
     return RGBA_F16;
   }
   return resource_provider_->best_texture_format();

gpu/command_buffer/service/gles2_cmd_decoder_passthrough.cc

@@ -1151,6 +1151,9 @@ gpu::Capabilities GLES2DecoderPassthroughImpl::GetCapabilities() {
   caps.texture_norm16 = feature_info_->feature_flags().ext_texture_norm16;
   caps.texture_half_float_linear =
       feature_info_->feature_flags().enable_texture_half_float_linear;
+  caps.color_buffer_half_float_rgba =
+      feature_info_->ext_color_buffer_float_available() ||
+      feature_info_->ext_color_buffer_half_float_available();
   caps.image_ycbcr_422 =
       feature_info_->feature_flags().chromium_image_ycbcr_422;
   caps.image_ycbcr_420v =