Only use fontMetrics if available in line-height computation

third_party/WebKit/Source/core/style/ComputedStyle.cpp

Index: third_party/WebKit/Source/core/style/ComputedStyle.cpp
diff --git a/third_party/WebKit/Source/core/style/ComputedStyle.cpp b/third_party/WebKit/Source/core/style/ComputedStyle.cpp
index 3852f28062f1f9c3cbc46d26e638ce95829b3e2b..6cef0714c8c54393249ab3155b0119353ac869b3 100644
--- a/third_party/WebKit/Source/core/style/ComputedStyle.cpp
+++ b/third_party/WebKit/Source/core/style/ComputedStyle.cpp
@@ -1307,8 +1307,9 @@ int ComputedStyle::computedLineHeight() const
 {
     const Length& lh = lineHeight();
 
-    // Negative value means the line height is not set. Use the font's built-in spacing.
-    if (lh.isNegative())
+    // Negative value means the line height is not set. Use the font's built-in
+    // spacing, if avalible.
+    if (lh.isNegative() && font().primaryFont())
         return fontMetrics().lineSpacing();
 
     if (lh.hasPercent())