Set MaxDrawBuffers to 1 if WEBGL_draw_buffers is not enabled in WebGL.

BUG=
TEST=webgl conformance tests
R=bajones@chromium.org, kbr@chromium.org

Review URL: https://codereview.chromium.org/23464034

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221316 0039d316-1c4b-4281-b951-d872f2087c98

gpu/command_buffer/service/gles2_cmd_decoder.cc

@@ -2483,6 +2483,8 @@ bool GLES2DecoderImpl::InitializeShaderTranslator() {
     resources.OES_standard_derivatives = derivatives_explicitly_enabled_;
     resources.EXT_frag_depth = frag_depth_explicitly_enabled_;
     resources.EXT_draw_buffers = draw_buffers_explicitly_enabled_;
+    if (!draw_buffers_explicitly_enabled_)
+      resources.MaxDrawBuffers = 1;
   } else {
     resources.OES_standard_derivatives =
         features().oes_standard_derivatives ? 1 : 0;