Create raster task for off-thread paint worklet

This CL creates a raster task for paint worklet. In this CL, we built
the framework only, therefore the task is an empty task. In the future,
the task will dispatch JS paint callbacks to the worklet thread and
produces the paint records.

We added a unit to make sure that the task is successfully created.

Bug: 907897
Change-Id: I9f45f9fac91b27263bde5beb0a18b7e0292dd1d3
Reviewed-on: https://chromium-review.googlesource.com/c/1348860
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617598}

• cc/BUILD.gn[diff]
• cc/tiles/image_controller.cc[diff]
• cc/tiles/image_controller.h[diff]
• cc/tiles/image_controller_unittest.cc[diff]
• cc/tiles/paint_worklet_image_cache.cc[Added - diff]
• cc/tiles/paint_worklet_image_cache.h[Added - diff]
• cc/tiles/paint_worklet_image_cache_unittest.cc[Added - diff]
• cc/tiles/tile_manager.cc[diff]