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This code previously lived in an android-specific directory, so it was
inappropriate to include it on other platforms. Now that we've moved
platform-agnostic code to chrome/browser/vr, the tab helper can also
be relocated, removing a number of unfortunate ifdefs.

Bug: 728421
Change-Id: I2b3f5e6b7369a751ed1e947f7882b2a8dd75e38e
Reviewed-on: https://chromium-review.googlesource.com/567327Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485972}