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This CL renames core => common, updates build deps to be a less
tangled, renames a few classes, and updates imports.

Change-Id: I011f9ec6d704681b50da5be759341e7e3b48d076
Bug: 897463
Reviewed-on: https://chromium-review.googlesource.com/c/1289688
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#605443}