Increase shard for browser_tests on win

Currently browser_tests on win7_chromium_rel_ng sometimes took more than 10 mins.
Let me increase the number of shards for such tests.

e.g
753s https://ci.chromium.org/b/8936390613165886176
732s https://ci.chromium.org/b/8936406831776876544
706s https://ci.chromium.org/b/8936396862731996176
704s https://ci.chromium.org/b/8936363432077079248

Bug: 868114
Change-Id: I9a7eb07ce2c58f5b03e5f39152dba7ac6101f11b
Reviewed-on: https://chromium-review.googlesource.com/1203519
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588764}
2 files changed