firstRectForCharacterRange(): don't fall over in an editable-less frame.

Source/web/WebLocalFrameImpl.cpp

Index: Source/web/WebLocalFrameImpl.cpp
diff --git a/Source/web/WebLocalFrameImpl.cpp b/Source/web/WebLocalFrameImpl.cpp
index 6413b4695320d07018765c5d942c8a462e6f3177..bdc3632a0d750da53129cb622ee23ed74d3d8420 100644
--- a/Source/web/WebLocalFrameImpl.cpp
+++ b/Source/web/WebLocalFrameImpl.cpp
@@ -1078,7 +1078,8 @@ bool WebLocalFrameImpl::firstRectForCharacterRange(unsigned location, unsigned l
         length = 0;
 
     Element* editable = frame()->selection().rootEditableElementOrDocumentElement();
-    ASSERT(editable);
+    if (!editable)
+        return false;
     RefPtrWillBeRawPtr<Range> range = PlainTextRange(location, location + length).createRange(*editable);
     if (!range)
         return false;