Fix crash when system retains an accessibility object.

content/browser/accessibility/browser_accessibility_cocoa.mm

Index: content/browser/accessibility/browser_accessibility_cocoa.mm
diff --git a/content/browser/accessibility/browser_accessibility_cocoa.mm b/content/browser/accessibility/browser_accessibility_cocoa.mm
index 545ed0ce50ed3fb9224eba9b58a8014600dd4d4e..1cd10daeb33520322379ff910201020ae3d94653 100644
--- a/content/browser/accessibility/browser_accessibility_cocoa.mm
+++ b/content/browser/accessibility/browser_accessibility_cocoa.mm
@@ -1421,6 +1421,8 @@ NSDictionary* attributeToMethodNameMap = nil;
 
   BrowserAccessibilityCocoa* hit = self;
   for (BrowserAccessibilityCocoa* child in [self children]) {
+    if (!child)

[dmazzoni, 2013/11/05 16:44:32]

This doesn't look right, I don't see how |child| should ever be null.

The issue fixed by the previous change was that BrowserAccessibilityCocoa's
browserAccessibility_ pointer becomes NULL - I don't see how we should ever have
a pointer to a NULL child...

+      continue;
     NSPoint origin = [child origin];
     NSSize size = [[child size] sizeValue];
     NSRect rect;