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Makes the mojo_core shared library a data dependency of the chrome
binary target on Chrome OS to ensure that it gets built whenever chrome
gets built. A follow-up change to chromite will cause this to be
included in device deployments.

Also adds a linker script to ensure that symbol exports are limited to
MojoGetSystemThunks.

Bug: 822034
Change-Id: Iad03f28377ecc5094c2019b22c723d7705242230
Reviewed-on: https://chromium-review.googlesource.com/1024750Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552901}