blink: Move MovableString to platform/bindings.

A subsequent CL will rename MovableString to a more suitable name, this
was split to minimize the diff.

Also fix trivial "git cl lint" warnings.

Bug: 877044
Change-Id: I2b96530bc227100e59d840f5abeb5e7995521af9
Reviewed-on: https://chromium-review.googlesource.com/1193442
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587526}
19 files changed