[TabStripModelRefactor] Add tab count to tabstripmodel tracing.

the "tabs" field got removed from the previous CL - https://chromium-review.googlesource.com/c/chromium/src/+/5602189. Since adding collections separates data structures now for contents_data_ and TabContentsData is instead used to represent both, replace it by tab_count.

Change-Id: I8aef9dfa0b5c23ee942c691132b7ed1d608fe5b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5608679


Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Shibalik Mohapatra <shibalik@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312154}