Add a command-line flag to enable threaded GPU rasterization.

cc/trees/layer_tree_settings.cc

 // Copyright 2011 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/trees/layer_tree_settings.h"
 
 #include <GLES2/gl2.h>
 #include <limits>
 
 #include "base/command_line.h"
@@ skipping 12 matching lines @@
       using_synchronous_renderer_compositor(false),
       disable_hi_res_timer_tasks_on_battery(false),
       report_overscroll_only_for_scrollable_axes(false),
       per_tile_painting_enabled(false),
       accelerated_animation_enabled(true),
       can_use_lcd_text(true),
       use_distance_field_text(false),
       gpu_rasterization_enabled(false),
       gpu_rasterization_forced(false),
       gpu_rasterization_msaa_sample_count(0),
+      threaded_gpu_rasterization_enabled(false),
       create_low_res_tiling(false),
       scrollbar_animator(NoAnimator),
       scrollbar_fade_delay_ms(0),
       scrollbar_fade_resize_delay_ms(0),
       scrollbar_fade_duration_ms(0),
       scrollbar_show_scale_threshold(1.0f),
       solid_color_scrollbar_color(SK_ColorWHITE),
       calculate_top_controls_position(false),
       timeout_and_draw_when_animation_checkerboards(true),
       maximum_number_of_failed_draws_before_draw_is_forced_(3),
@@ skipping 26 matching lines @@
       scheduled_raster_task_limit(32),
       use_occlusion_for_tile_prioritization(false),
       record_full_layer(false),
       use_display_lists(false),
       verify_property_trees(false) {
 }
 
 LayerTreeSettings::~LayerTreeSettings() {}
 
 }  // namespace cc