Don't crash when using IME on an input field with a CSS-specified font.

BUG=471689
TEST=repro in bug 473859 no longer crashes

Review URL: https://codereview.chromium.org/1052523006

Cr-Commit-Position: refs/heads/master@{#323971}

content/common/mac/attributed_string_coder.mm

@@ -100,10 +100,12 @@ AttributedStringCoder::FontAttribute::FontAttribute()
 AttributedStringCoder::FontAttribute::~FontAttribute() {
 }
 
-NSDictionary* AttributedStringCoder::FontAttribute::ToAttributesDictionary(
-    void) const {
+NSDictionary*
+AttributedStringCoder::FontAttribute::ToAttributesDictionary() const {
   DCHECK(ShouldEncode());
   NSFont* font = font_descriptor_.ToNSFont();
+  if (!font)
+    return [NSDictionary dictionary];
   return [NSDictionary dictionaryWithObject:font forKey:NSFontAttributeName];
 }
 

content/common/mac/font_descriptor.h

@@ -21,7 +21,9 @@ struct CONTENT_EXPORT FontDescriptor {
 
   FontDescriptor() : font_point_size(0) {}
 
-  // Return an autoreleased NSFont corresponding to the font description.
+  // Return an autoreleased NSFont corresponding to the font description. Note
+  // that this will return nil if the font specified isn't available in this
+  // process.
   NSFont* ToNSFont() const;
 
   // Name of the font.