Increase shards of content_browsertests on linux-chromeos-dbg

The shard run time were close to the time out limit of 1h, causing
frequent failures.

Increase the number of shards to 16.

Note that this builder isn't on the CQ. So capacity impact should
be negligible.

BUG=b:343500256

Change-Id: I120d9bbf8453921ee62ce38e625dfcedf8e75856
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5619671


Commit-Queue: Alvin Jia <alvinjia@google.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313691}