Add extensions_unittests suite to more memory bots.

testing/buildbot/chromium_memory_trybot.json

 {
   "gtest_tests": [
     {
       "test": "accessibility_unittests",
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
     },
     {
       "test": "base_unittests",
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
     },
@@ skipping 28 matching lines @@
     {
       "test": "device_unittests",
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
     },
     {
       "test": "display_unittests",
       "platforms": ["linux"],
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
     },
     {
+      "test": "extensions_unittests",
+      "platforms": ["linux", "win", "mac"],

[James Cook, 2014/09/23 16:16:38]

dpranke, does he need this list?  extensions_unittests can also run on Chrome
OS, but not Android or iOS.  I'm guessing a blank platforms list == linux, mac,
chromeos, maybe win?

+      "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
+    },
+    {
       "test": "gcm_unit_tests",
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
     },
     {
       "test": "gfx_unittests",
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
     },
     {
       "test": "google_apis_unittests",
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
@@ skipping 54 matching lines @@
     {
       "test": "unit_tests",
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
     },
     {
       "test": "url_unittests",
       "exclude_builders": ["tryserver.chromium.linux:linux_browser_asan"]
     }
   ]
 }