Try to do less work clearing the font-cache in StyleEngine::didDetach.

Clearing the whole font-cache without considering css-connected fonts
could be cheaper. This is a speculative fix for issue 677415.

BUG=677415

Review-Url: https://codereview.chromium.org/2622673003
Cr-Commit-Position: refs/heads/master@{#443202}

third_party/WebKit/Source/core/dom/StyleEngine.cpp

@@ -451,7 +451,9 @@ void StyleEngine::didDetach() {
   m_activeTreeScopes.clear();
   m_viewportResolver = nullptr;
   m_mediaQueryEvaluator = nullptr;
-  clearFontCache();
+  if (m_fontSelector)
+    m_fontSelector->fontFaceCache()->clearAll();
+  m_fontSelector = nullptr;
 }
 
 void StyleEngine::clearFontCache() {