Implement EXT_draw_buffers WebGL extention support in command buffer.

gpu/command_buffer/service/context_group.cc

Index: gpu/command_buffer/service/context_group.cc
===================================================================
--- gpu/command_buffer/service/context_group.cc	(revision 186778)
+++ gpu/command_buffer/service/context_group.cc	(working copy)
@@ -46,6 +46,8 @@
       max_fragment_uniform_vectors_(0u),
       max_varying_vectors_(0u),
       max_vertex_uniform_vectors_(0u),
+      max_color_attachments_(1u),
+      max_draw_buffers_(1u),
       program_cache_(NULL),
       feature_info_(new FeatureInfo()) {
   {
@@ -185,6 +187,11 @@
     max_vertex_uniform_vectors_ /= 4;
   }
 
+  if (feature_info_->feature_flags().ext_draw_buffers) {
+    GetIntegerv(GL_MAX_COLOR_ATTACHMENTS_EXT, &max_color_attachments_);
+    GetIntegerv(GL_MAX_DRAW_BUFFERS_ARB, &max_draw_buffers_);
+  }
+
   const GLint kMinFragmentUniformVectors = 16;
   const GLint kMinVaryingVectors = 8;
   const GLint kMinVertexUniformVectors = 128;