Ensure TabGroupUnderline bounds are always non-zero

See inline comments for why this is needed and when it happens.

I don't have a good test repro for this, since I'd need to manually trigger a paint cycle to get the underline to appear anyway. I was, however, able to reproduce the issue in the bug and verify that it is fixed.

Bug: 1014913
Change-Id: I0b70be1b0d83370714a004b96a437ca589c34bf1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874448Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Connie Wan <connily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709687}

chrome/browser/ui/views/tabs/tab_group_underline.cc

@@ -22,7 +22,11 @@ constexpr int TabGroupUnderline::kStrokeThickness;
 
 TabGroupUnderline::TabGroupUnderline(TabStrip* tab_strip, TabGroupId group)
     : tab_strip_(tab_strip), group_(group) {
-  UpdateBounds();
+  // Set non-zero bounds to start with, so that painting isn't pruned.
+  // Needed because UpdateBounds() happens during OnPaint(), which is called
+  // after painting is pruned.
+  const int y = tab_strip_->bounds().height() - 1;
+  SetBounds(0, y - kStrokeThickness, kStrokeThickness * 2, kStrokeThickness);
 }
 
 void TabGroupUnderline::OnPaint(gfx::Canvas* canvas) {
@@ -40,6 +44,12 @@ void TabGroupUnderline::UpdateBounds() {
   const int start_x = GetStart();
   const int end_x = GetEnd();
 
+  // The width may be zero if the group underline and header are initialized at
+  // the same time, as with tab restore. In this case, don't update the bounds
+  // and defer to the next paint cycle.
+  if (end_x <= start_x)
+    return;
+
   const int start_y = tab_strip_->bounds().height() - 1;
 
   SetBounds(start_x, start_y - kStrokeThickness, end_x - start_x,