Enable v8.runtimestats.browse_mobile* benchmarks on reference builds.

Enable v8.rutimestats.browse_mobile and v8.runtimestats.browse_mobile_classic
benchmarks on reference builds. They were earlier disabled because these
benchmarks were failing on Nexus5 and Nexus6 bots.

Bug: 
Change-Id: Ia7819a540be28d802734fbc3e34c9c08d45428aa
Reviewed-on: https://chromium-review.googlesource.com/490287Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#468597}

tools/perf/benchmarks/v8_browsing.py

@@ -279,7 +279,6 @@ class V8RuntimeStatsDesktopClassicBrowsingBenchmark(
 
 
 
-@benchmark.Disabled('reference')  # http://crbug.com/694658
 @benchmark.Enabled('android')
 @benchmark.Owner(emails=['mythria@chromium.org'])
 class V8RuntimeStatsMobileBrowsingBenchmark(
@@ -291,7 +290,6 @@ class V8RuntimeStatsMobileBrowsingBenchmark(
     return 'v8.runtimestats.browsing_mobile'
 
 
-@benchmark.Disabled('reference')  # http://crbug.com/694658
 @benchmark.Enabled('android')
 @benchmark.Disabled('all')
 @benchmark.Owner(emails=['mythria@chromium.org'])
@@ -309,7 +307,6 @@ class V8RuntimeStatsMobileTurboBrowsingBenchmark(
     return 'v8.runtimestats.browsing_mobile_turbo'
 
 
-@benchmark.Disabled('reference')  # http://crbug.com/694658
 @benchmark.Enabled('android')
 @benchmark.Owner(emails=['hablich@chromium.org'])
 class V8RuntimeStatsMobileClassicBrowsingBenchmark(