Fix compilation for ATK (accessibility)

A variable wasn't given the right name inside the ATK support when
a patch consolidated 3 AX properties into single restrictions
property for accessibility objects.

R=aleventhal@chromium.org, jochen@chromium.org

Change-Id: I0c82e055e39614110cfc4f7a7299d226bd4e6ad2
Reviewed-on: https://chromium-review.googlesource.com/580927Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#488630}

content/browser/accessibility/browser_accessibility_auralinux.cc

@@ -571,7 +571,7 @@ static AtkStateSet* browser_accessibility_ref_state_set(AtkObject* atk_object) {
 // it's best to leave this out rather than break people's builds:
 #if defined(ATK_CHECK_VERSION)
 #if ATK_CHECK_VERSION(2, 16, 0)
-      atk_state_set_add_state(atk_state_set, ATK_STATE_READ_ONLY);
+      atk_state_set_add_state(state_set, ATK_STATE_READ_ONLY);
 #endif
 #endif
       break;