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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}