Add GL_EXT_color_buffer_half_float support

gpu/command_buffer/service/framebuffer_manager.cc

Index: gpu/command_buffer/service/framebuffer_manager.cc
diff --git a/gpu/command_buffer/service/framebuffer_manager.cc b/gpu/command_buffer/service/framebuffer_manager.cc
index 63912958a87ebbe2d25f91401d13cdd6d7959438..55ed2e1a1f8e1bec6ef38e86b8c8bea46d9b11df 100644
--- a/gpu/command_buffer/service/framebuffer_manager.cc
+++ b/gpu/command_buffer/service/framebuffer_manager.cc
@@ -269,6 +269,10 @@ class TextureAttachment
         internal_format == GL_LUMINANCE_ALPHA) {
       return false;
     }
+    // Disallow RGB16F, because it is only supported in ES2 and not ES3.
+    if (internal_format == GL_RGB16F) {
+      return false;
+    }
     return (need & have) != 0;
   }