Shard Win 7 Tests x64 (1) to 6 instead of 3 machines.

The Win 7 x64 testers had twice as many cores as the 32 bit ones, and I didn't notice that when picking the sharding level earlier. Now that they're all going to the same swarming slaves, which are almost all 4-core, increase the sharding level.

BUG=414808
R=maruel@chromium.org

Review URL: https://codereview.chromium.org/619673003

Cr-Commit-Position: refs/heads/master@{#297437}

testing/buildbot/chromium.win.json

@@ -245,7 +245,7 @@
         "test": "browser_tests",
         "swarming": {
           "can_use_on_swarming_builders": true,
-          "shards": 3
+          "shards": 6
         }
       },
       "app_list_unittests",