Avoid string duplication in GetTimeHistogram.

Pass string by const-ref instead of by const value. This is likely
what was intended but for a missing `&`, as callers already have
a const-ref.

-- avoid making temporary string from literal.
-- stop short of using base::StrCat() to avoid another temporary.

Change-Id: I8aa50249ac234e1d3297beb5ca528cfedb1521e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5893047
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Sky Malice <skym@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1360869}