cc: Replace some instances of unordered_map with base::flat_map.

cc/output/gl_renderer.h

 // Copyright 2010 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.
 
 #ifndef CC_OUTPUT_GL_RENDERER_H_
 #define CC_OUTPUT_GL_RENDERER_H_
 
 #include <deque>
+#include <unordered_map>
 #include <vector>
 
 #include "base/cancelable_callback.h"
 #include "base/macros.h"
 #include "cc/cc_export.h"
 #include "cc/output/color_lut_cache.h"
 #include "cc/output/context_cache_controller.h"
 #include "cc/output/direct_renderer.h"
 #include "cc/output/gl_renderer_draw_cache.h"
 #include "cc/output/program_binding.h"
@@ skipping 273 matching lines @@
   unsigned offscreen_framebuffer_id_ = 0u;
 
   std::unique_ptr<StaticGeometryBinding> shared_geometry_;
   std::unique_ptr<DynamicGeometryBinding> clipped_geometry_;
   gfx::QuadF shared_geometry_quad_;
 
   // This will return nullptr if the requested program has not yet been
   // initialized.
   const Program* GetProgramIfInitialized(const ProgramKey& key) const;
 
   std::unordered_map<ProgramKey, std::unique_ptr<Program>, ProgramKeyHash>

[vmpstr, 2017/04/26 22:12:00]

The ProgramKey here is pretty big. It does have an equality operator, but
writing an operator< for it is a bit awkward (especially since one of the
comparison operators compares a pointer). 

Is the guidance in these cases to bite the bullet and just write operator<?

[danakj, 2017/04/26 22:16:31]

 2017/04/26 22:12:00, vmpstr wrote:
As long as it's consistent and transitive and such, I don't think that comparing
pointers is a problem.

However I don't think we really understood the performance of flat_map with
large keys. As long as you're considering that size when you look at the size of
the map, and not just the number of elements then it should be okay I think
though. You might wanna perftest to be sure for this one tho, it does seem like
new territory.

       program_cache_;
 
   const gfx::ColorTransform* GetColorTransform(const gfx::ColorSpace& src,
                                                const gfx::ColorSpace& dst);
   std::map<gfx::ColorSpace,
            std::map<gfx::ColorSpace, std::unique_ptr<gfx::ColorTransform>>>
       color_transform_cache_;
 
   gpu::gles2::GLES2Interface* gl_;
   gpu::ContextSupport* context_support_;
@@ skipping 53 matching lines @@
   gfx::Size offscreen_stencil_renderbuffer_size_;
 
   base::WeakPtrFactory<GLRenderer> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(GLRenderer);
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_GL_RENDERER_H_