shelf: Separate back button and app list button.

Seperate the back button and app list button into separate elements.
Draws the background in ShelfView::OnPaint. This fixes issues with ink
drop and accessibility focusing not working correctly. Made changes to
ShelfView focus search to accommodate this.

Add back_button_unittest.
Removed app_list_button/back button ink drop unittest which tested
if different parts of the old app list button's ink drop acted
different if different parts were pressed. Not needed as the two
separate buttons' ink drops should work normally.
Removed app_list_button rtl unittest as the buttons should be flipped
normally by the views rtl handling.

Test: ash_unittests *Shelf* *ApplistButton* *BackButton*
Bug: 791638, 758402
Change-Id: I07bdd0b490ffb2619a13cebacf388e2e08190114
Reviewed-on: https://chromium-review.googlesource.com/761856
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524301}

• ash/BUILD.gn[diff]
• ash/ash_strings.grd[diff]
• ash/metrics/user_metrics_recorder.cc[diff]
• ash/metrics/user_metrics_recorder_unittest.cc[diff]
• ash/public/cpp/shelf_model.cc[diff]
• ash/public/cpp/shelf_model.h[diff]
• ash/public/cpp/shelf_model_unittest.cc[diff]
• ash/public/cpp/shelf_struct_traits.h[diff]
• ash/public/cpp/shelf_types.cc[diff]
• ash/public/cpp/shelf_types.h[diff]
• ash/public/interfaces/shelf.mojom[diff]
• ash/resources/vector_icons/shelf_back.1x.icon[diff]
• ash/resources/vector_icons/shelf_back.icon[diff]
• ash/shelf/app_list_button.cc[diff]
• ash/shelf/app_list_button.h[diff]
• ash/shelf/app_list_button_unittest.cc[diff]
• ash/shelf/assistant_overlay.cc[diff]
• ash/shelf/back_button.cc[Added - diff]
• ash/shelf/back_button.h[Added - diff]
• ash/shelf/back_button_unittest.cc[Added - diff]
• ash/shelf/shelf.cc[diff]
• ash/shelf/shelf_controller.cc[diff]
• ash/shelf/shelf_controller_unittest.cc[diff]
• ash/shelf/shelf_unittest.cc[diff]
• ash/shelf/shelf_view.cc[diff]
• ash/shelf/shelf_view.h[diff]
• ash/shelf/shelf_view_test_api.cc[diff]
• ash/shelf/shelf_view_unittest.cc[diff]
• ash/shelf/shelf_window_watcher_unittest.cc[diff]
• chrome/browser/ui/ash/launcher/chrome_launcher_controller.cc[diff]
• chrome/browser/ui/ash/launcher/chrome_launcher_controller_browsertest.cc[diff]
• chrome/browser/ui/ash/launcher/chrome_launcher_controller_unittest.cc[diff]
• chrome/browser/ui/window_sizer/window_sizer_ash_uitest.cc[diff]