Update local BaseScrollBar::contents_scroll_offset_ on Update.

ui/views/controls/scrollbar/scrollbar_unittest.cc

Index: ui/views/controls/scrollbar/scrollbar_unittest.cc
diff --git a/ui/views/controls/scrollbar/scrollbar_unittest.cc b/ui/views/controls/scrollbar/scrollbar_unittest.cc
index cdf256a6207925b023a1a1874ff420604faf6e1e..c3c96df9b038dcf3115893f30d177cc3f0b46289 100644
--- a/ui/views/controls/scrollbar/scrollbar_unittest.cc
+++ b/ui/views/controls/scrollbar/scrollbar_unittest.cc
@@ -156,4 +156,15 @@ TEST_F(NativeScrollBarTest, MAYBE_ScrollBarFitsToBottom) {
       scrollbar_->GetPosition());
 }
 
+TEST_F(NativeScrollBarTest, ScrollToEndAfterShrinkAndExpand) {
+  // Scroll to the end of the content.
+  scrollbar_->Update(100, 101, 0);
+  EXPECT_TRUE(scrollbar_->ScrollByContentsOffset(-1));
+  // Shrink and then re-exapnd the content.
+  scrollbar_->Update(100, 100, 0);
+  scrollbar_->Update(100, 101, 0);
+  // Ensure the scrollbar allows scrolling to the end.
+  EXPECT_TRUE(scrollbar_->ScrollByContentsOffset(-1));
+}
+
 }  // namespace views