mash cleanup: Remove unused window title shown property

I performed a quick codesearch audit from the PropertyMirror removal CL:
http://crrev.com/c/1603215/3/ash/public/cpp/mus_property_mirror_ash.cc#b75
kTitleShownKey is the only obviously unused property.

Bug: 961478
Test: No behavior change
Change-Id: I43fcd9d5968c4fba4aa8a4a2af45b5f9f9372366
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1666559
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670358}

ui/aura/client/aura_constants.cc

@@ -76,7 +76,6 @@ DEFINE_UI_CLASS_PROPERTY_KEY(ui::WindowShowState,
                              kShowStateKey,
                              ui::SHOW_STATE_DEFAULT)
 DEFINE_OWNED_UI_CLASS_PROPERTY_KEY(base::string16, kTitleKey, nullptr)
-DEFINE_UI_CLASS_PROPERTY_KEY(bool, kTitleShownKey, true)
 DEFINE_UI_CLASS_PROPERTY_KEY(int, kTopViewInset, 0)
 DEFINE_OWNED_UI_CLASS_PROPERTY_KEY(gfx::ImageSkia, kWindowIconKey, nullptr)
 DEFINE_UI_CLASS_PROPERTY_KEY(int, kWindowCornerRadiusKey, -1)

ui/aura/client/aura_constants.h

@@ -155,10 +155,6 @@ AURA_EXPORT extern const WindowProperty<ui::WindowShowState>* const
 // A property key to store the title of the window; sometimes shown to users.
 AURA_EXPORT extern const WindowProperty<base::string16*>* const kTitleKey;
 
-// Indicates if the title of the window should be shown. This is only used for
-// top-levels that show a title. Default is false.
-AURA_EXPORT extern const WindowProperty<bool>* const kTitleShownKey;
-
 // The inset of the topmost view in the client view from the top of the
 // non-client view. The topmost view depends on the window type. The topmost
 // view is the tab strip for tabbed browser windows, the toolbar for popups,