Change timeout of mash/mus_browser_Tests to 15 minutes

They should not come close to one hour.

BUG=717028
TEST=test only change
R=jonross@chromium.org
TBR=jonross@chromium.org

Review-Url: https://codereview.chromium.org/2855623002
Cr-Commit-Position: refs/heads/master@{#468324}

testing/buildbot/chromium.chromiumos.json

@@ -251,7 +251,7 @@
           "--override-use-software-gl-for-tests",
           "--test-launcher-filter-file=../../testing/buildbot/filters/mash.browser_tests.filter"
         ],
-        "hard_timeout": 60,
+        "hard_timeout": 15,
         "swarming": {
           "can_use_on_swarming_builders": true
         },
@@ -299,7 +299,7 @@
           "--override-use-software-gl-for-tests",
           "--test-launcher-filter-file=../../testing/buildbot/filters/mus.browser_tests.filter"
         ],
-        "hard_timeout": 60,
+        "hard_timeout": 15,
         "swarming": {
           "can_use_on_swarming_builders": true
         },