This is a reland of https://codereview.chromium.org/1502373009 with some fixes
for components/metrics/leak_detector allocator type mismatches.

Original issue's description:
> Allow std::unordered_*.
>
> base::hash_* is, as a transition step, implemented in terms of
> std::unordered_*. Later commits will convert existing uses.
>
> Also fix a host of IWYU problems that arose from this CL.
>
> (NOPRESUBMIT because the wstring presubmit check is overzealous
> and complains about the reference to wstring in the comment.)
>
> Committed: https://crrev.com/3f37f7f1459e7b5a452c0e433493e0a6e9649ca7
> Cr-Commit-Position: refs/heads/master@{#370553}

BUG=576864
TBR=derat@chromium.org,danakj@chromium.org,dalecurtis@chromium.org,jbauman@chromium.org,blundell@chromium.org
NOPRESUBMIT=true
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1615713003

Cr-Commit-Position: refs/heads/master@{#370867}