Avoid <picture> preloading when viewport not initialized.

third_party/WebKit/Source/core/html/parser/HTMLPreloadScanner.cpp

Index: third_party/WebKit/Source/core/html/parser/HTMLPreloadScanner.cpp
diff --git a/third_party/WebKit/Source/core/html/parser/HTMLPreloadScanner.cpp b/third_party/WebKit/Source/core/html/parser/HTMLPreloadScanner.cpp
index 8884bdda02babbf08b12cce11402e8f9fc123491..1f44cd5c50a5d364c85fe3455c0473e5367d7858 100644
--- a/third_party/WebKit/Source/core/html/parser/HTMLPreloadScanner.cpp
+++ b/third_party/WebKit/Source/core/html/parser/HTMLPreloadScanner.cpp
@@ -807,7 +807,11 @@ void TokenPreloadScanner::scanCommon(const Token& token,
 
       StartTagScanner scanner(tagImpl, m_mediaValues);
       scanner.processAttributes(token.attributes());
-      if (m_inPicture)
+      // TODO(yoav): ViewportWidth is currently racy and might be zero in some
+      // cases, at least in tests.

[Charlie Harrison, 2016/10/10 19:23:12]

nit: Fix comment reflow by removing line breaks.

+      // That problem will go away once ParseHTMLOnMainThread lands and
+      // MediaValuesCached is eliminated.
+      if (m_inPicture && m_mediaValues->viewportWidth())
         scanner.handlePictureSourceURL(m_pictureData);
       std::unique_ptr<PreloadRequest> request = scanner.createPreloadRequest(
           m_predictedBaseElementURL, source, m_clientHintsPreferences,