viz: Fix a content_browsertests.

ScrollLatencyBrowserTest.SmoothWheelScroll waits for an UMA metric to
be collected. With OOP-D, the metric is collected in the gpu process,
instead of in the browser process. So, make sure the test collects
all the UMA from the child processes so that it can see the UMA it is
waiting for.

BUG=786151

Change-Id: I58d93c32c4ee57b1f8ba24de1a370bac97f0e17f
Reviewed-on: https://chromium-review.googlesource.com/959454
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542622}

content/browser/renderer_host/input/scroll_latency_browsertest.cc

@@ -128,6 +128,7 @@ IN_PROC_BROWSER_TEST_F(ScrollLatencyBrowserTest, SmoothWheelScroll) {
   size_t num_samples = 0;
 
   while (num_samples == 0) {
+    FetchHistogramsFromChildProcesses();
     num_samples =
         histogram_tester
             .GetAllSamples(

testing/buildbot/filters/viz.content_browsertests.filter

@@ -3,10 +3,6 @@
 # http://crbug.com/784941
 -CaptureScreenshotTest.*
 
-#### ui::LatencyInfo
-# Fails to receive histogram updates http://crbug.com/786151
--ScrollLatencyBrowserTest.*
-
 # OnGpuSwapBuffersCompletedInternal not called. http://crbug.com/791557
 -MouseLatencyBrowserTest.CoalescedMouseMovesCorrectlyTerminated