Implement EXT_draw_buffers WebGL extention support in command buffer.

gpu/command_buffer/common/gles2_cmd_utils.cc

Index: gpu/command_buffer/common/gles2_cmd_utils.cc
===================================================================
--- gpu/command_buffer/common/gles2_cmd_utils.cc	(revision 187725)
+++ gpu/command_buffer/common/gles2_cmd_utils.cc	(working copy)
@@ -622,15 +622,19 @@
   }
 }
 
-uint32 GLES2Util::GetChannelsNeededForAttachmentType(int type) {
+uint32 GLES2Util::GetChannelsNeededForAttachmentType(
+    int type, uint32 max_color_attachments) {
   switch (type) {
-    case GL_COLOR_ATTACHMENT0:
-      return kRGBA;
     case GL_DEPTH_ATTACHMENT:
       return kDepth;
     case GL_STENCIL_ATTACHMENT:
       return kStencil;
     default:
+      if (type >= GL_COLOR_ATTACHMENT0 &&
+          type < static_cast<int>(
+              GL_COLOR_ATTACHMENT0 + max_color_attachments)) {
+        return kRGBA;
+      }
       return 0x0000;
   }
 }