Reland "Tab strip only occupies the space needed for its tabs."

This is a reland of b6c0ca2a

The only change is a line in tab_scrubber_browsertest.cc that ensures
that any invalid layout are laid out after modifying the tabstrip.

Original change's description:
> Tab strip only occupies the space needed for its tabs.
>
> Bug: 1093972
> Change-Id: I0ca6cc1a7dad9240b85745fefb39ac4f52eb9355
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300993
> Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
> Reviewed-by: Connie Wan <connily@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#793270}

Bug: 1093972
Change-Id: I4ded68fde336b14b38ad862cd63118ebc9fc7a72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2336296
Commit-Queue: Taylor Bergquist <tbergquist@chromium.org>
Reviewed-by: Connie Wan <connily@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797853}

chrome/browser/ui/ash/tab_scrubber_browsertest.cc

@@ -224,6 +224,10 @@ class TabScrubberTest : public InProcessBrowserTest,
     ASSERT_EQ(num_tabs, browser->tab_strip_model()->active_index());
     tab_strip->StopAnimating(true);
     ASSERT_FALSE(tab_strip->IsAnimating());
+    // Perform any scheduled layouts so the tabstrip is in a steady state.
+    BrowserView::GetBrowserViewForBrowser(browser)
+        ->GetWidget()
+        ->LayoutRootViewIfNecessary();
   }
 
   // TabStripModelObserver overrides.

chrome/browser/ui/views/frame/tab_strip_region_view.cc

@@ -10,15 +10,28 @@
 #include "ui/views/layout/flex_layout.h"
 #include "ui/views/view_class_properties.h"
 
+namespace {
+
+// Size calculation used for tabstrip scroll container. Equivalent to using
+// a (kPreferredScaleToMinimum, kPreferred) flex specification on the tabstrip
+// itself, bypassing the ScrollView.
+gfx::Size TabScrollContainerFlexRule(const views::View* tab_strip,
+                                     const views::View* view,
+                                     const views::SizeBounds& size_bounds) {
+  const gfx::Size preferred_size = tab_strip->GetPreferredSize();
+  int width = preferred_size.width();
+  if (size_bounds.width().has_value())
+    width = std::min(width, size_bounds.width().value());
+  return gfx::Size(width, preferred_size.height());
+}
+
+}  // namespace
+
 TabStripRegionView::TabStripRegionView(std::unique_ptr<TabStrip> tab_strip) {
   views::FlexLayout* layout_manager =
       SetLayoutManager(std::make_unique<views::FlexLayout>());
 
-  layout_manager->SetOrientation(views::LayoutOrientation::kHorizontal)
-      .SetDefault(
-          views::kFlexBehaviorKey,
-          views::FlexSpecification(views::MinimumFlexSizeRule::kScaleToZero,
-                                   views::MaximumFlexSizeRule::kUnbounded));
+  layout_manager->SetOrientation(views::LayoutOrientation::kHorizontal);
 
   tab_strip_ = tab_strip.get();
   tab_strip->SetAvailableWidthCallback(
@@ -31,8 +44,16 @@ TabStripRegionView::TabStripRegionView(std::unique_ptr<TabStrip> tab_strip) {
     tab_strip_scroll_container->SetHideHorizontalScrollBar(true);
     tab_strip_container_ = tab_strip_scroll_container;
     tab_strip_scroll_container->SetContents(std::move(tab_strip));
+    tab_strip_scroll_container->SetProperty(
+        views::kFlexBehaviorKey,
+        views::FlexSpecification(base::BindRepeating(
+            &TabScrollContainerFlexRule, base::Unretained(tab_strip_))));
   } else {
     tab_strip_container_ = AddChildView(std::move(tab_strip));
+    tab_strip_->SetProperty(
+        views::kFlexBehaviorKey,
+        views::FlexSpecification(views::MinimumFlexSizeRule::kScaleToZero,
+                                 views::MaximumFlexSizeRule::kPreferred));
   }
 }
 

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

@@ -254,6 +254,10 @@ void TabDragControllerTest::AddTabsAndResetBrowser(Browser* browser,
   }
   browser->window()->Show();
   StopAnimating(GetTabStripForBrowser(browser));
+  // Perform any pending layouts happen so the tabstrip is in a steady state.
+  BrowserView::GetBrowserViewForBrowser(browser)
+      ->GetWidget()
+      ->LayoutRootViewIfNecessary();
   ResetIDs(browser->tab_strip_model(), 0);
 }
 

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

@@ -576,7 +576,8 @@ class TabStrip::TabDragContextImpl : public TabDragContext {
   int GetTabAreaWidth() const override { return tab_strip_->GetTabAreaWidth(); }
 
   int GetTabDragAreaWidth() const override {
-    return tab_strip_->width() - tab_strip_->FrameGrabWidth();
+    return std::max(tab_strip_->CalculateAvailableWidthForTabs(),
+                    tab_strip_->GetTabAreaWidth());
   }
 
   int TabDragAreaBeginX() const override {