mac: Add tests for reading images from NSPasteboard.

ui/base/clipboard/clipboard_mac.mm

Index: ui/base/clipboard/clipboard_mac.mm
diff --git a/ui/base/clipboard/clipboard_mac.mm b/ui/base/clipboard/clipboard_mac.mm
index 8c42c0b7bd99a252a7a6cf0cae6b56dfcfcb2ef6..202a7f9ba948a86b2dede34128ac05ce8f37ae86 100644
--- a/ui/base/clipboard/clipboard_mac.mm
+++ b/ui/base/clipboard/clipboard_mac.mm
@@ -298,7 +298,7 @@ void ClipboardMac::ReadRTF(ClipboardType type, std::string* result) const {
   return ReadData(GetRtfFormatType(), result);
 }
 
-SkBitmap ClipboardMac::ReadImage(ClipboardType type) const {
+SkBitmap ClipboardMac::ReadImage(ClipboardType type, NSPasteboard* pb) const {
   DCHECK(CalledOnValidThread());
   DCHECK_EQ(type, CLIPBOARD_TYPE_COPY_PASTE);
 
@@ -306,7 +306,6 @@ SkBitmap ClipboardMac::ReadImage(ClipboardType type) const {
   // may throw, and that exception will leak. Prevent a crash in that case;
   // a blank image is better.
   base::scoped_nsobject<NSImage> image;
-  NSPasteboard* pb = GetPasteboard();
   @try {
     if ([[pb types] containsObject:NSFilenamesPboardType]) {
       // -[NSImage initWithPasteboard:] gets confused with copies of a single
@@ -333,6 +332,10 @@ SkBitmap ClipboardMac::ReadImage(ClipboardType type) const {
   return bitmap;
 }
 
+SkBitmap ClipboardMac::ReadImage(ClipboardType type) const {
+  return ReadImage(type, GetPasteboard());
+}
+
 void ClipboardMac::ReadCustomData(ClipboardType clipboard_type,
                                   const base::string16& type,
                                   base::string16* result) const {