cc: Simplify Task and its derived classes.

This patch merges ImageDecodeTask and RasterTask into TileTask, single
class defining properties specific to tasks used for tiles. Now all
the tile related tasks get derived directly from TileTask.

  RasterTaskImpl : TileTask
  ImageDecodeTaskImpl : TileTask
  ImageUploadTaskImpl : TileTask

This patch also splits (Task + TaskGraph) from TaskGraphRunner and
moves it to its own file which simplifies file include dependencies.

BUG=599863
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1890903002

Cr-Commit-Position: refs/heads/master@{#388152}

• cc/BUILD.gn[diff]
• cc/cc.gyp[diff]
• cc/raster/task.cc[Renamed from cc/raster/task_graph_runner.cc - diff]
• cc/raster/task.h[Added - diff]
• cc/raster/task_graph_runner.h[diff]
• cc/raster/task_graph_work_queue.cc[diff]
• cc/raster/tile_task_runner.cc[diff]
• cc/raster/tile_task_runner.h[diff]
• cc/raster/tile_task_worker_pool.h[diff]
• cc/raster/tile_task_worker_pool_perftest.cc[diff]
• cc/raster/tile_task_worker_pool_unittest.cc[diff]
• cc/test/fake_tile_manager.cc[diff]
• cc/tiles/gpu_image_decode_controller.cc[diff]
• cc/tiles/gpu_image_decode_controller.h[diff]
• cc/tiles/gpu_image_decode_controller_unittest.cc[diff]
• cc/tiles/image_decode_controller.h[diff]
• cc/tiles/software_image_decode_controller.cc[diff]
• cc/tiles/software_image_decode_controller.h[diff]
• cc/tiles/software_image_decode_controller_unittest.cc[diff]
• cc/tiles/tile.h[diff]
• cc/tiles/tile_manager.cc[diff]
• cc/tiles/tile_manager.h[diff]
• cc/trees/layer_tree_host_impl.h[diff]