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The build target is used to separate dependency to //contents in
ui/keyboard, but that dependency was removed. We don't have any reason
to have the separate build target anymore.

This CL does:
- Remove 'keyboard_with_content" build target.
- Merge files in ui/keyboard/content into ui/keyboard/keyboard_resource_util.
- Remove redundant includes.

Test: Unit tests pass.
Bug: None
Change-Id: Ibdb47c4c170381da28f4aaad8cc606dbaab9f687
Reviewed-on: https://chromium-review.googlesource.com/970102
Commit-Queue: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544625}