If JavaScript shows a dialog, cause the page to lose fullscreen.

BUG=670135, 550017, 726761, 728276

Review-Url: https://codereview.chromium.org/2906133004
Cr-Commit-Position: refs/heads/master@{#478884}
  explicit AXTreeSnapshotCombiner(AXTreeSnapshotCallback callback)
      : callback_(callback) {
  }

  explicit AXTreeSnapshotCombiner(AXTreeSnapshotCallback callback)
      : callback_(callback) {
  }
