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This makes it clear that this target depends on //content and lays the
groundwork for a future sessions_ios target.

The files in components/sessions/core/ are moved to a variable so that
they can be shared across targets.

TBR=finnur@chromium.org
TBR=sky@chromium.org
BUG=371476
TEST=None

Review URL: https://codereview.chromium.org/673843002

Cr-Commit-Position: refs/heads/master@{#301103}