[Mac] Force-enable the find next/previous buttons when pressing

Cmd-F to repeat a search.

BUG=http://crbug.com/18762
BUG=http://crbug.com/20273
TEST=See test cases in bugs.  Both should work now.
Review URL: http://codereview.chromium.org/200050

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@25722 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/cocoa/find_bar_cocoa_controller.mm

@@ -128,6 +128,12 @@
 
 - (void)setFocusAndSelection {
   [[findText_ window] makeFirstResponder:findText_];
+
+  // Enable the buttons if the find text is non-empty.
+  BOOL buttonsEnabled = ([[findText_ stringValue] length] > 0) ? YES : NO;
+  [previousButton_ setEnabled:buttonsEnabled];
+  [nextButton_ setEnabled:buttonsEnabled];
+
 }
 
 - (void)setFindText:(const string16&)findText {