Prefetch @import files from CSS files.

Source/core/html/HTMLLinkElement.cpp

Index: Source/core/html/HTMLLinkElement.cpp
diff --git a/Source/core/html/HTMLLinkElement.cpp b/Source/core/html/HTMLLinkElement.cpp
index 3ee6600b7302235de6b0ff3d4cefcf801407c2f1..dfdd1bd416601b9bb0dbed3839d415652f090659 100644
--- a/Source/core/html/HTMLLinkElement.cpp
+++ b/Source/core/html/HTMLLinkElement.cpp
@@ -436,6 +436,8 @@ PassOwnPtrWillBeRawPtr<LinkStyle> LinkStyle::create(HTMLLinkElement* owner)
 
 LinkStyle::LinkStyle(HTMLLinkElement* owner)
     : LinkResource(owner)
+    , m_preloadScanner(adoptPtr(new CSSPreloadScanner))
+    , m_preloadInput(adoptPtr(new SegmentedString))
     , m_disabledState(Unset)
     , m_pendingSheetType(None)
     , m_loading(false)
@@ -502,6 +504,23 @@ void LinkStyle::setCSSStyleSheet(const String& href, const KURL& baseURL, const
         const_cast<CSSStyleSheetResource*>(cachedStyleSheet)->saveParsedStyleSheet(styleSheet);
 }
 
+void LinkStyle::dataReceived(const CSSStyleSheetResource* cachedStyleSheet, const char* data, int length)
+{
+    if (!m_preloadScanner)
+        return;
+
+    PreloadRequestStream pendingPreloads;
+    m_preloadInput->append(SegmentedString(data));

[abarth-chromium, 2014/06/26 16:37:59]

Why don't we need to use |length| on this line?

+    m_preloadScanner->scan(data, *m_preloadInput, pendingPreloads);
+    HTMLResourcePreloader m_preloader(&document());
+    m_preloader.takeAndPreload(pendingPreloads);
+
+    if (m_preloadScanner->isDoneParsingImports()) {

[eseidel, 2014/05/28 23:52:52]

Could you explain the magic here a bit more?

// Note:  Although dataRecieved will be called for each packet of data returned
from the network
// @import statements always are at the top of the CSS file.  As soon as we
reach
// a part of the CSS file which is not an @import we can delete our preloader
and ignore
// the rest of file.  If we ever make CSS preloading more complicated
// (to look for URLs other than @imports) we will need to re-think this logic.

+        m_preloadScanner.clear();
+        m_preloadInput.clear();
+    }
+}
+
 bool LinkStyle::sheetLoaded()
 {
     if (!styleSheetIsLoading()) {