Do not run startup.mobile on webview bots

The benchmark cannot report metrics correctly for WebView. Also WebView
cares about different startup scenarios and has its own stories for
that.

Bug: 880091
Change-Id: I76bec3c24843a5429bfeec45a53439b19b3d7ef6
Reviewed-on: https://chromium-review.googlesource.com/1217103Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590252}

tools/perf/benchmarks/startup_mobile.py

@@ -162,7 +162,7 @@ class _MobileStartupStorySet(story_module.StorySet):
 @benchmark.Info(emails=['pasko@chromium.org',
                         'chrome-android-perf-status@chromium.org'])
 class MobileStartupBenchmark(perf_benchmark.PerfBenchmark):
-  SUPPORTED_PLATFORMS = [story_module.expectations.ALL_MOBILE]
+  SUPPORTED_PLATFORMS = [story_module.expectations.ANDROID_NOT_WEBVIEW]
 
   # Set |pageset_repeat| to 1 to control the amount of iterations from the
   # stories. This would avoid setting per-story pageset_repeat at bisect time.