[Mac] Fix accessibility crash because I am terrible with dynamic types.

Turns out attributes like AXPosition will crash with @[] :/

BUG=412707
TEST=Start Chrome with --enable-new-avatar-menu. Provided you have zooming
enabled in your Mac Accessibility settings, press Ctrl and scroll up to zoom
in. Click on the avatar button. The avatar bubble should open and Chrome should
not crash.

Review URL: https://codereview.chromium.org/645933002

Cr-Commit-Position: refs/heads/master@{#299160}

chrome/browser/ui/cocoa/profiles/profile_chooser_controller.mm

@@ -780,7 +780,7 @@ class ActiveProfileObserverBridge : public AvatarMenuObserver,
 }
 
 - (id)accessibilityAttributeValue:(NSString*)attribute {
-  return @[];
+  return nil;
 }
 
 - (BOOL)canBecomeKeyView {
@@ -1159,7 +1159,7 @@ class ActiveProfileObserverBridge : public AvatarMenuObserver,
   // Add a dummy, empty element so that we don't initially display any
   // focus rings.
   NSButton* dummyFocusButton =
-      [[[DummyWindowFocusButton alloc] initWithFrame:NSZeroRect] autorelease];
+     [[[DummyWindowFocusButton alloc] initWithFrame:NSZeroRect] autorelease];
   [dummyFocusButton setNextKeyView:subView];
   [[self window] makeFirstResponder:dummyFocusButton];