Fix BrowserAccessibilityManager initialization race.

I think this regressed due to:
https://codereview.chromium.org/1119923002

BUG=486851

Review URL: https://codereview.chromium.org/1141573003

Cr-Commit-Position: refs/heads/master@{#329492}

content/browser/accessibility/browser_accessibility_manager_android.cc

@@ -71,8 +71,8 @@ BrowserAccessibilityManagerAndroid::BrowserAccessibilityManagerAndroid(
     BrowserAccessibilityDelegate* delegate,
     BrowserAccessibilityFactory* factory)
     : BrowserAccessibilityManager(delegate, factory) {
-  SetContentViewCore(content_view_core);
   Initialize(initial_tree);
+  SetContentViewCore(content_view_core);
 }
 
 BrowserAccessibilityManagerAndroid::~BrowserAccessibilityManagerAndroid() {