[Mac][Material Design] Use white tab spinner for dark themes.

In Material Design we display a white tab spinner for Incognito mode
tabs, but we need to also do so for dark themes.

R=avi@chromium.org
BUG=624106

Review-Url: https://codereview.chromium.org/2103073003
Cr-Commit-Position: refs/heads/master@{#402643}

chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm

@@ -1564,7 +1564,7 @@ private:
   } else if (contents->IsWaitingForResponse()) {
     newState = kTabWaiting;
     if (ui::MaterialDesignController::IsModeMaterial() &&
-        [[[tabController view] window] inIncognitoMode]) {
+        [[[tabController view] window] hasDarkTheme]) {
       throbberImage = throbberWaitingIncognitoImage;
     } else {
       throbberImage = throbberWaitingImage;
@@ -1572,7 +1572,7 @@ private:
   } else if (contents->IsLoadingToDifferentDocument()) {
     newState = kTabLoading;
     if (ui::MaterialDesignController::IsModeMaterial() &&
-        [[[tabController view] window] inIncognitoMode]) {
+        [[[tabController view] window] hasDarkTheme]) {
       throbberImage = throbberLoadingIncognitoImage;
     } else {
       throbberImage = throbberLoadingImage;