base: ThreadGroup: Optimize away allocations

Recording the histogram results in 2 useless allocations around
std::string concatenation. Move the string creation to the ctor as the
string is anyways only used once.

This showed up on Instruments as an individual big contributor for PA
to require thread synchronization (return to central allocator). While
this doesn't eliminate the need for overall PA synchronization it
eliminates this one useless trigger.

Fixed: 445765268
Change-Id: If50bb4fc2d9d5fba1df32b26ca6a08de6ecc767d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6964796
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1517215}