FirstMeaningfulPaint UMA: Reduce the network activity threshold

This patch changes the duration FirstMeaningfulPaintDetector waits
before reporting FirstMeaningfulPaint from 2.0sec to 0.5sec.

This is safe because only 0.18% of FirstMeaningfulPaint is in the range
of [network stable - 2sec, network stable - 0.5sec].

BUG=632081
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2345443002
Cr-Commit-Position: refs/heads/master@{#419638}

chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc

@@ -432,7 +432,7 @@ IN_PROC_BROWSER_TEST_F(PageLoadMetricsBrowserTest,
                                embedded_test_server()->GetURL("/title1.html"));
 
   // Wait until the renderer finishes observing layouts.
-  const int kNetworkIdleTime = 2000;
+  const int kNetworkIdleTime = 500;
   const int kMargin = 500;
   const std::string javascript = base::StringPrintf(
       "setTimeout(() => window.domAutomationController.send(true), %d)",

third_party/WebKit/Source/core/paint/FirstMeaningfulPaintDetector.cpp

@@ -19,7 +19,7 @@ const int kBlankCharactersThreshold = 200;
 
 // FirstMeaningfulPaintDetector stops observing layouts and reports First
 // Meaningful Paint when this duration passed from last network activity.
-const double kSecondsWithoutNetworkActivityThreshold = 2.0;
+const double kSecondsWithoutNetworkActivityThreshold = 0.5;
 
 } // namespace