Safe Browsing: Add ScopedBlockingCall(MAY_BLOCK) before remote call

The CL also adds a microsecond histogram to the call, to see how often
we'd get above the 10ms threshold in practice. A similar histogram
exists (SubresourceFilter.SafeBrowsing.CheckDispatchTime), but it is
only called for SubresourceFilter safe browsing checks on main frame
navigations.

Bug: 806981
Change-Id: If63590a4f543a1e65760ee5c7f35d8bc773cef10
Reviewed-on: https://chromium-review.googlesource.com/911750
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535954}
2 files changed