Run 32 bit android_webview_test_apk on bots with only a single device.

Both with 8 shards and 16min timeout.

Example release shard:
https://chromium-swarm.appspot.com/task?id=375199b32dd47210

Example debug shard:
https://chromium-swarm.appspot.com/task?id=37519aa55a5dae10

Will move some *-b4 bots over to the 1-device pool if necessary.

Bug: 670879
Change-Id: I3b49ba2c34f37bc3ad2d283294d41e5dce4949d7
Reviewed-on: https://chromium-review.googlesource.com/569004Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Yoland Yan <yolandyan@chromium.org>
Commit-Queue: Benjamin Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486501}

testing/buildbot/chromium.linux.json

@@ -1588,7 +1588,7 @@
           ],
           "dimension_sets": [
             {
-              "android_devices": "6",
+              "android_devices": "1",
               "device_os": "KTU84P",
               "device_type": "hammerhead"
             }
@@ -1603,7 +1603,8 @@
               ],
               "name": "shard #${SHARD_INDEX} logcats"
             }
-          ]
+          ],
+          "shards": 8
         },
         "test": "webview_instrumentation_test_apk"
       }
@@ -3228,7 +3229,7 @@
           ],
           "dimension_sets": [
             {
-              "android_devices": "6",
+              "android_devices": "1",
               "device_os": "KTU84P",
               "device_type": "hammerhead"
             }
@@ -3243,7 +3244,8 @@
               ],
               "name": "shard #${SHARD_INDEX} logcats"
             }
-          ]
+          ],
+          "shards": 8
         },
         "test": "webview_instrumentation_test_apk"
       }