Do not ignore SolidColorScrollbarLayer's scroll_layer_id ctor argument

cc/layers/solid_color_scrollbar_layer.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layers/solid_color_scrollbar_layer.h"
 
 #include <memory>
 
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/solid_color_scrollbar_layer_impl.h"
@@ skipping 20 matching lines @@
   return make_scoped_refptr(new SolidColorScrollbarLayer(
       orientation, thumb_thickness, track_start,
       is_left_side_vertical_scrollbar, scroll_layer_id));
 }
 
 SolidColorScrollbarLayer::SolidColorScrollbarLayerInputs::
     SolidColorScrollbarLayerInputs(ScrollbarOrientation orientation,
                                    int thumb_thickness,
                                    int track_start,
                                    bool is_left_side_vertical_scrollbar,
                                    int scroll_layer_id)

[ajuma, 2017/03/04 00:04:50]

It looks like this gets called in non-test code with scroll_layer_id hardcoded
to 0: 
https://cs.chromium.org/chromium/src/cc/blink/web_scrollbar_layer_impl.cc?rcl...

The same happens for the other scrollbar layer subclasses (that do use the
scroll_layer_id argument) though. But it seems confusing, since 0 is not the
same as INVALID_ID (which is -1).

Would it make sense as part of this cleanup to change the callsites that are
passing in 0 when they mean INVALID_ID to actually use INVALID_ID?

[pdr., 2017/03/04 02:44:23]

We can actually do even better: these callsites are not needed at all. I have a
followup patch to remove them which cleans up a bunch of code:
https://codereview.chromium.org/2728253002

Mind if we land this patch as-is?

-    : scroll_layer_id(Layer::INVALID_ID),
+    : scroll_layer_id(scroll_layer_id),
       orientation(orientation),
       thumb_thickness(thumb_thickness),
       track_start(track_start),
       is_left_side_vertical_scrollbar(is_left_side_vertical_scrollbar) {}
 
 SolidColorScrollbarLayer::SolidColorScrollbarLayerInputs::
     ~SolidColorScrollbarLayerInputs() = default;
 
 SolidColorScrollbarLayer::SolidColorScrollbarLayer(
     ScrollbarOrientation orientation,
@@ skipping 52 matching lines @@
 
   solid_color_scrollbar_layer_inputs_.scroll_layer_id = layer_id;
   SetNeedsFullTreeSync();
 }
 
 ScrollbarOrientation SolidColorScrollbarLayer::orientation() const {
   return solid_color_scrollbar_layer_inputs_.orientation;
 }
 
 }  // namespace cc