mac: Make RWHVMac::ClearCompositorFrame clear locks

Ensure that the BrowserCompositorMac not hold on to a compositor lock
when requested to clear its compositor frame. This lock may be held
indefinitely (if the renderer hangs) and so the frame will never be
cleared.

Bug: 739621
Change-Id: I15d0e82bdf632f3379a48e959f198afb8a4ac218
Reviewed-on: https://chromium-review.googlesource.com/608239
Commit-Queue: ccameron chromium <ccameron@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493563}
void DelegatedFrameHost::OnFirstSurfaceActivation(
    const viz::SurfaceInfo& surface_info) {
}

void DelegatedFrameHost::OnFirstSurfaceActivation(
    const viz::SurfaceInfo& surface_info) {
}
