DevTools: force a style recaclc before reporting final CSS coverage

Without this, taking a coverage report could be flaky if nothing
forced a style recalc. This can happen very often in headless mode.

Change-Id: If2bb596736f274d0aeceabec9ff7cf6076886eae
Reviewed-on: https://chromium-review.googlesource.com/c/1279293
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599782}

third_party/WebKit/LayoutTests/inspector-protocol/css/css-coverage-new-stylesheet-expected.txt

+Test that CSS coverage works for a newly added stylesheet.
+Successfully reported CSS coverage.
+

third_party/WebKit/LayoutTests/inspector-protocol/css/css-coverage-new-stylesheet.js

+(async function(testRunner) {
+  const {session, dp} = await testRunner.startBlank('Test that CSS coverage works for a newly added stylesheet.');
+
+  await dp.DOM.enable();
+  await dp.CSS.enable();
+
+  await dp.CSS.startRuleUsageTracking();
+  await session.evaluateAsync(async function(url) {
+    const div = document.createElement('div');
+    div.classList.add('usedAtTheVeryEnd');
+    document.body.appendChild(div);
+
+    const link = document.createElement('link');
+    link.rel = 'stylesheet';
+    link.href = url;
+    document.head.appendChild(link);
+    await new Promise(fulfill => link.onload = fulfill);
+  }, testRunner.url('./resources/coverage2.css'));
+  const response = await dp.CSS.stopRuleUsageTracking();
+
+  if (response.result.ruleUsage.length === 1) {
+    testRunner.log('Successfully reported CSS coverage.')
+  } else {
+    testRunner.log(`ERROR!`);
+    testRunner.log(response.result.ruleUsage);
+  }
+  testRunner.completeTest();
+});

third_party/blink/renderer/core/inspector/inspector_css_agent.cc

@@ -2421,6 +2421,8 @@ Response InspectorCSSAgent::startRuleUsageTracking() {
 
 Response InspectorCSSAgent::stopRuleUsageTracking(
     std::unique_ptr<protocol::Array<protocol::CSS::RuleUsage>>* result) {
+  for (Document* document : dom_agent_->Documents())
+    document->UpdateStyleAndLayoutTree();
   Response response = takeCoverageDelta(result);
   SetCoverageEnabled(false);
   return response;