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Currently, base::bits::Count{Leading,Trailing}ZeroBits fail to link
when used with 64-bit inputs on 32-bit MSVC builds because MSVC only
implements the _BitScan{Forward,Reverse}64 intrinsics for 64-bit
builds. Fix this by implementing equivalent support using 32-bit
intrinsics.

Change-Id: I9b9095abd03587c29dd642df65cf9783bd9b202d
Reviewed-on: https://chromium-review.googlesource.com/c/1321253Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607887}