Decrease sharding of *content_browsertets in cros.

The content_browsertests and non_viz_contentbrowsertests run
3x-5x faster than browser_tests and non_viz_browser_tests
at the 95th percentile on linux-chromeos-rel. On debug, the
pattern is still there, but less pronounced.

Decreasing sharding by ~1/2 on both test suites should help
return some capacity without affectingi runtimes.

Bug: 1025798
Change-Id: I5a5004c92f12c7f0c9f77f2b3ed5679e1ed89d85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1923031
Commit-Queue: Albert J. Wong <ajwong@chromium.org>
Auto-Submit: Albert J. Wong <ajwong@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716474}