[Sheriff] Double the shards for content_browsertests on Linux ASan Tests (sandboxed)

As per https://crbug.com/1051944#c2

Bug: 1051944
Change-Id: I93e399580f1b4e4da6e7f3317602a9f2b591244a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057047Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741445}

testing/buildbot/chromium.memory.json

@@ -4801,7 +4801,7 @@
               "os": "Ubuntu-16.04"
             }
           ],
-          "shards": 2
+          "shards": 4
         },
         "test": "content_browsertests",
         "test_target": "//content/test:content_browsertests"

testing/buildbot/test_suite_exceptions.pyl

@@ -419,7 +419,7 @@
       },
       'Linux ASan Tests (sandboxed)': {
         'swarming': {
-          'shards': 2, # https://crbug.com/955853
+          'shards': 4, # https://crbug.com/1051944
         },
       },
       'Linux TSan Tests': {