[ui] use default member initializer for some members in MockDrawableData

This is followup for
https://chromium-review.googlesource.com/c/chromium/src/+/1750477

Change-Id: Ib1151a14b482d015985e5d0bc4f5cb00898dfafe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752262
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Mohsen Izadi <mohsen@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686429}

ui/touch_selection/touch_handle_unittest.cc

@@ -22,8 +22,8 @@ const gfx::RectF kDefaultViewportRect(0, 0, 560, 1200);
 struct MockDrawableData {
   TouchHandleOrientation orientation = TouchHandleOrientation::UNDEFINED;
   float alpha = 0.f;
-  bool mirror_horizontal;
-  bool mirror_vertical;
+  bool mirror_horizontal = false;
+  bool mirror_vertical = false;
   bool enabled = false;
   bool visible = false;
   gfx::RectF rect{0, 0, kDefaultDrawableSize, kDefaultDrawableSize};