Increase shards level for browser_tests on Linux ChromiumOS Tests (dbg)(1)

Changes:
Linux ChromiumOS Tests (dbg) is slower than release bots. So try to increase the number of shards here to see if recent timeout exceptions on crbug.com/686265 could be fixed.

BUG=686265
TEST=NONE

Review-Url: https://codereview.chromium.org/2658203004
Cr-Commit-Position: refs/heads/master@{#446887}
1 file changed