[tools/perf] Disable browse:chrome:newtab on webview

Webview does not have new tab page.

TBR=nednguyen@google.com,ssid@chromium.org
BUG=716041

Review-Url: https://codereview.chromium.org/2848893002
Cr-Commit-Position: refs/heads/master@{#468095}

tools/perf/page_sets/system_health/searching_stories.py

@@ -70,6 +70,7 @@ class SearchOmniboxStory(system_health_story.SystemHealthStory):
     action_runner.ScrollPage(use_touch=True, distance=500)
 
 
+@decorators.Disabled('android-webview')  # Webview does not have new tab page.
 class MobileNewTabPageStory(system_health_story.SystemHealthStory):
   """Story that loads new tab page and performs searches.