Update buttons' state when FindPboard is updated

When global FindPboard is updated, find result is cleared
and next/prev buttons are disabled. But this makes
users unabled to find the exact string without
editing text. So enable buttons unless the text from pboard
is empty.

Bug: 739735
Change-Id: Iad72ae1e0a88165774fecd21fe26c3a2949c60c4
Reviewed-on: https://chromium-review.googlesource.com/888229Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532459}

chrome/browser/ui/cocoa/find_bar/find_bar_cocoa_controller.mm

@@ -187,6 +187,10 @@ const int kUndefinedResultCount = -1;
   if (!suppressPboardUpdateActions_)
     [self prepopulateText:[[FindPasteboard sharedInstance] findText]];
   [self clearFindResultsForCurrentBrowser];
+
+  BOOL buttonsEnabled = [[findText_ stringValue] length] > 0 ? YES : NO;
+  [previousButton_ setEnabled:buttonsEnabled];
+  [nextButton_ setEnabled:buttonsEnabled];
 }
 
 - (void)positionFindBarViewAtMaxY:(CGFloat)maxY maxWidth:(CGFloat)maxWidth {