Fix use of uninitialized ptr in FloatingMenuButton

This change fixes an uninitialized pointer in the FloatingMenuButton
class that triggers MemorySanitizer errors on bots.

Currently, if FloatingMenuButton is initialized via its default
constructor, its member `VectorIcon* icon_` is uninitialized. This
causes errors when the value of icon_ is checked in SetVectorIcon().

Bug: 772945
Bug: 938501
Bug: 1130078
Bug: 1146255
Change-Id: I21f9596f496d6fd5090bd2e307dc648117be07ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523477
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825288}

ash/system/accessibility/floating_menu_button.h

@@ -68,7 +68,7 @@ class FloatingMenuButton : public views::ImageButton {
  private:
   void UpdateImage();
 
-  const gfx::VectorIcon* icon_;
+  const gfx::VectorIcon* icon_ = nullptr;
   // True if the button is currently toggled.
   bool toggled_ = false;
   int size_ = kTrayItemSize;