[PinnedTabs] Do not record pinned only metric before strip initialization

We have more than 50k recording[1] of this metric, which is unusually high, and the reason is due to we'd record this before initialization where mWidth is still 0 and #getAvailableTabWidthForResizing() is a negative number, because `#getAvailableTabWidthForResizing(-40) = mWidth(0) - mLeftMargin(0) - mRightMargin`(static value 40dp NTB).

[1]: https://uma.googleplex.com/p/chrome/variations?sid=31aba6a78ee4cc043bb3104613a9ed5d

Bug: 444068018
Change-Id: Icbe0348a443104807ebd7eda8ae7d17dfd99a389
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7173563
Reviewed-by: Neil Coronado <nemco@google.com>
Auto-Submit: Zhe Li <zheliooo@google.com>
Reviewed-by: Wenyu Fu <wenyufu@chromium.org>
Commit-Queue: Wenyu Fu <wenyufu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1547927}