Enable the Rightware Browsermark benchmark

This patch un-disables the rightware browsermark benchmark. Manually
testing shows this finishes in under 5m on a Nexus 5.

BUG=423018

Review URL: https://codereview.chromium.org/649283002

Cr-Commit-Position: refs/heads/master@{#299369}

tools/perf/benchmarks/browsermark.py

@@ -46,7 +46,6 @@ class _BrowsermarkMeasurement(page_test.PageTest):
         scalar.ScalarValue(results.current_page, 'Score', 'score', result))
 
 
-@benchmark.Disabled
 class Browsermark(benchmark.Benchmark):
   """Browsermark suite tests CSS, DOM, resize, page load, WebGL and JS."""
   test = _BrowsermarkMeasurement