[StartSurface] Set incognito description height to match parent

This CL fixes the bug in
https://chromium-review.googlesource.com/c/chromium/src/+/2243334

Without setting the height to match_parent, it will be unscrollable
when its content is short.

Screen record before
https://drive.google.com/file/d/1CUOz6ErGL2fxkF2pGAC_fGWEcTGy8hrD/view?usp=sharing

Screen record after
https://drive.google.com/file/d/1CeuBtjQbGRovJC8H0Vk-wY6wpWqmlQJ0/view?usp=sharing

Bug: 1093506
Change-Id: Iff9df7ef0050885c02fe55676dbfa2046d6e78a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2253143
Commit-Queue: Ganggui Tang <gogerald@chromium.org>
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Auto-Submit: Ganggui Tang <gogerald@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780086}

chrome/android/features/tab_ui/java/res/layout/tasks_view_layout.xml

@@ -78,7 +78,7 @@
     <ViewStub
         android:id="@+id/incognito_description_container_layout_stub"
         android:layout_width="match_parent"
-        android:layout_height="wrap_content"
+        android:layout_height="match_parent"
         android:layout="@layout/incognito_description_container_layout"
         app:layout_behavior="@string/appbar_scrolling_view_behavior" />