Increase shards for surface_sync_content_browsertests

This is for slow surface_sync_content_browsertests step like belows.
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/android-marshmallow-arm64-rel/54921
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/android-marshmallow-arm64-rel/54740
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/android-marshmallow-arm64-rel/54965

Overhead of the test is small, 6 seconds overhead vs 35 mins runtime.
https://chromium-swarm.appspot.com/task?id=3f2773404bdad410&refresh=10&show_raw=1

So overhead increase is negligible.

Bug: 868114
Change-Id: Iab6b396fcdf5a25ba50f299a0fa5e294ccea32ae
Reviewed-on: https://chromium-review.googlesource.com/1163642
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581069}
6 files changed