Make components_unittests build with string16=std::u16string.

Commonly, this requires making Windows-API-specific code more consistent
about using std::wstring instead of assuming it's the same as string16.

Bug: 911896
Change-Id: I05cc9bb1ff851bcbf9b5a770dfb446f0fc187804
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2661802


Reviewed-by: Satoru Takabayashi <satorux@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Wei Li <weili@chromium.org>
Reviewed-by: Filip Gorski <fgorski@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#849267}
GitOrigin-RevId: 31fa0f2aa8747674276d61cd4915d11a4e764422