Initialize variable to nullptr in accessibility_manager.cc.

MSAN started complaining on uninitializing this variable (the first fail
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20ChromiumOS%20MSan%20Tests/9484).
Probably the immediate culprit is
https://chromium-review.googlesource.com/c/1320027, but it's hard to say.
Anyway this issue was earlier and it's easier to fix it, by initializing it to null then revert the CL.

TBR=dtseng@chromium.org
TBR=dmazzoni@chromium.org

Change-Id: Ie045adfcf6860b47c8e649432c6b6c6abf2061b4
Reviewed-on: https://chromium-review.googlesource.com/c/1323090Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606044}

chrome/browser/chromeos/accessibility/accessibility_manager.cc

@@ -239,6 +239,7 @@ AccessibilityManager::AccessibilityManager()
       scoped_braille_observer_(this),
       braille_ime_current_(false),
       chromevox_panel_(nullptr),
+      switch_access_panel_(nullptr),
       extension_registry_observer_(this),
       weak_ptr_factory_(this) {
   notification_registrar_.Add(this,