accessibility: Do not update cursor visibility for touch exploration

BEFORE: Touch exploration causes the mouse cursor to appear even if
there are no pointing devices whatsoever.
AFTER: Touch exploration has no effect on cursor visibility.

Bug: b/167952435
Test: manual
Change-Id: I0ff8d0ddf59272183bed65655f10be5aeb57210b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436627Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812230}

chrome/browser/accessibility/accessibility_extension_api.cc

@@ -343,6 +343,7 @@ AccessibilityPrivateSendSyntheticMouseEventFunction::Run() {
 
   int changed_button_flags = flags;
 
+  flags |= ui::EF_IS_SYNTHESIZED;
   if (mouse_data->touch_accessibility && *(mouse_data->touch_accessibility))
     flags |= ui::EF_TOUCH_ACCESSIBILITY;