Skip validate draw buffers when RASTERIZER_DISCARD is enabled.

Bug: 962555, 962636
Change-Id: I959445bef4bbc94f8e8b96c813205a49ab59419f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610606Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>
Cr-Commit-Position: refs/heads/master@{#659235}

gpu/command_buffer/service/gles2_cmd_decoder.cc

@@ -9972,6 +9972,9 @@ bool GLES2DecoderImpl::SupportsDrawBuffers() const {
 }
 
 bool GLES2DecoderImpl::ValidateAndAdjustDrawBuffers(const char* func_name) {
+  if (state_.GetEnabled(GL_RASTERIZER_DISCARD)) {
+    return true;
+  }
   if (!SupportsDrawBuffers()) {
     return true;
   }