Bump content_browsertests timeout to 20 minutes.

testing/buildbot/chromium.linux.json

 {
   "Android Arm64 Builder (dbg)": {
     "additional_compile_targets": [
       "android_builder_tests"
     ]
   },
   "Android Builder": {
     "additional_compile_targets": [
       "cronet_test_instrumentation_apk",
       "system_webview_apk"
@@ skipping 196 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
               "device_os": "KTU84P",
               "device_type": "hammerhead"
             }
           ],
-          "hard_timeout": 960
+          "hard_timeout": 1200
         },
         "test": "content_browsertests"
       },
       {
         "test": "content_unittests"
       },
       {
         "test": "device_unittests"
       },
       {
@@ skipping 1264 matching lines @@
         "name": "nacl_integration",
         "script": "nacl_integration.py"
       }
     ]
   },
   "Linux Tests (dbg)(1)(32)": {
     "gtest_tests": [],
     "scripts": []
   }
 }