And add a helper method to convert an uint32 value to uintptr (e.g. a
noop on 32 bit and Uint32ToUint64 conversion on 64 bit).

R=ahaas@chromium.org

Change-Id: Ibc6731bc7ddaf8ceaa27e9e8fbec916d184d9ad4
Reviewed-on: https://chromium-review.googlesource.com/836618
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50246}