[M-62] Restore forward declaration of MediaTrackConstraints

The cherry-pick in 19765bd888949485b1e7ee2892b6502b1a653025 removed the
forward declaration of MediaTrackConstraints from ImageCapture.h. This
was fine on head but this symbol was still used by another function
prototype on branch 3202.

Bug: 759457
Change-Id: Iba689e0e8222cb742d7eb2a8762e4b555bc3a256
Reviewed-on: https://chromium-review.googlesource.com/710531
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/branch-heads/3202@{#649}
Cr-Branched-From: fa6a5d87adff761bc16afc5498c3f5944c1daa68-refs/heads/master@{#499098}
1 file changed