Initial changes for TabGridDialog component.

This change contains the following:
* Introduce the skeleton for the TabGridDialog component.
* Expose TabGridDialog component to GridTabSwitcher.
* Introduce GridCardClickListener to trigger the opening of
  TabGridDialog.
* This feature is hidden behind "Tab Groups UI Improvement" flag.

Bug: 956205
Change-Id: Iaa7acf6cca278b35987300f0584e49dc41744f36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1582526
Commit-Queue: Yue Zhang <[email protected]>
Reviewed-by: Wei-Yin Chen (陳威尹) <[email protected]>
Reviewed-by: Yusuf Ozuysal <[email protected]>
Cr-Commit-Position: refs/heads/master@{#655656}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 032284e5..8450a5b7 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1104,6 +1104,9 @@
 extern const char kTabGroupsAndroidName[];
 extern const char kTabGroupsAndroidDescription[];
 
+extern const char kTabGroupsUiImprovementsAndroidName[];
+extern const char kTabGroupsUiImprovementsAndroidDescription[];
+
 extern const char kTabGroupsName[];
 extern const char kTabGroupsDescription[];