[StartSurface] Set correct initial tint for the tab switcher button

Before in dark mode:
https://drive.google.com/file/d/1uy3w9ooqoRyJ5GeSMbLLfncgg2rqUH6I/view?usp=sharing

after:
https://drive.google.com/file/d/1Mq6JRxglRYmFgxlX0ZOaCKiTV84KwmGf/view?usp=sharing
https://drive.google.com/file/d/1gZJvVCE3xaJUmbkGr9A9LzH9KF12buWp/view?usp=sharing

Bug: 1076449
Change-Id: I3ecaa14fa2c196a16df9005cf38884023ec7230c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2202100Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Ganggui Tang <gogerald@chromium.org>
Auto-Submit: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768826}

chrome/android/java/src/org/chromium/chrome/browser/toolbar/TabSwitcherButtonCoordinator.java

@@ -80,6 +80,8 @@ public class TabSwitcherButtonCoordinator {
             }
         };
         mThemeColorProvider.addTintObserver(mTintObserver);
+        mTabSwitcherButtonModel.set(
+                TabSwitcherButtonProperties.TINT, mThemeColorProvider.getTint());
     }
 
     public void setTabCountProvider(TabCountProvider tabCountProvider) {