Enable page_cycler_v2 and loading.mobile on webview

https://crrev.com/2558743002 fixed a bug that loading metric doesn't
work for webview traces. This patch re-enables page_cycler_v2 and
loading.mobile benchmarks on webview bots.

BUG=651188
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.perf:linux_perf_cq;master.tryserver.chromium.perf:mac_retina_perf_cq;master.tryserver.chromium.perf:winx64_10_perf_cq

Review-Url: https://codereview.chromium.org/2564773002
Cr-Commit-Position: refs/heads/master@{#437822}

tools/perf/benchmarks/loading.py

@@ -30,10 +30,6 @@ class LoadingMobile(perf_benchmark.PerfBenchmark):
     if possible_browser.browser_type == 'reference':
       return True
 
-    # crbug.com/651188
-    if possible_browser.browser_type == 'android-webview':
-      return True
-
     return False
 
   @classmethod

tools/perf/benchmarks/page_cycler_v2.py

@@ -59,12 +59,6 @@ class _PageCyclerV2(perf_benchmark.PerfBenchmark):
         possible_browser.platform.GetDeviceTypeName() == 'AOSP on BullHead'):
       return True
 
-    # crbug.com/651188
-    if ((possible_browser.platform.GetDeviceTypeName() == 'Nexus 6' or
-         possible_browser.platform.GetDeviceTypeName() == 'AOSP on Shamu') and
-        possible_browser.browser_type == 'android-webview'):
-      return True
-
     return False