chromeos: increase number of shards for mash_browsertests

The number of shards for browser_tests on
"Linux Chromium OS ASan LSan Tests (1)" was increased to 30 a while back,
but not mash_browser_tests. This means mash_browser_tests often timeout
on the bot. I'm upping the number of shards to be the same for the two.

BUG=794372
TEST=none

Change-Id: I24dd03bb0c61a65d34dfb290af1269026f9563a3
Reviewed-on: https://chromium-review.googlesource.com/1153549Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578768}

testing/buildbot/chromium.memory.json

@@ -4296,7 +4296,7 @@
         "swarming": {
           "can_use_on_swarming_builders": true,
           "hard_timeout": 1800,
-          "shards": 20
+          "shards": 30
         },
         "test": "browser_tests"
       },

testing/buildbot/test_suite_exceptions.pyl

@@ -752,8 +752,10 @@
       },
       # chromium.memory
       'Linux Chromium OS ASan LSan Tests (1)': {
+        # These are very slow on the ASAN trybot for some reason.
+        # crbug.com/794372
         'swarming': {
-          'shards': 20,
+          'shards': 30,
         },
       },
       'Linux ChromiumOS MSan Tests': {