Implement EXT_draw_buffers WebGL extention support in command buffer.

gpu/command_buffer/service/context_group.h

Index: gpu/command_buffer/service/context_group.h
===================================================================
--- gpu/command_buffer/service/context_group.h	(revision 186778)
+++ gpu/command_buffer/service/context_group.h	(working copy)
@@ -103,6 +103,14 @@
     return max_vertex_uniform_vectors_;
   }
 
+  uint32 max_color_attachments() const {
+    return max_color_attachments_;
+  }
+
+  uint32 max_draw_buffers() const {
+    return max_draw_buffers_;
+  }
+
   FeatureInfo* feature_info() {
     return feature_info_.get();
   }
@@ -175,6 +183,8 @@
   uint32 max_fragment_uniform_vectors_;
   uint32 max_varying_vectors_;
   uint32 max_vertex_uniform_vectors_;
+  uint32 max_color_attachments_;
+  uint32 max_draw_buffers_;
 
   ProgramCache* program_cache_;