Refactor coordinate system of OpaqueBrowserFrameViewLayout::trailing_button_start_

This CL is a refactor and has no behavioural changes.

OpaqueBrowserFrameView::trailing_button_start_ is an x coordinate
relative to the right edge of the window title bar.

This CL replaces trailing_button_start_ with available_space_trailing_x_
where
available_space_trailing_x_ == host->width() - trailing_button_start_.

available_space_trailing_x_ represents the same x position as
trailing_button_start_ except it's relative to the left edge of the
titlebar (0).

The name "button_start" has been replaced with "available_space_*_x"
as it's used for more than just button elements and now represents
a typical x co-ordinate.

This refactor is intended to support
https://chromium-review.googlesource.com/c/chromium/src/+/1156117
by making HostedAppButtonContainer::LayoutInContainer() less cumbersome
to use in OpaqueBrowserFrameViewLayout.

Bug: 854479
Change-Id: Iff1c29139cd4c8c66286db85fce85b99c94c5261
Reviewed-on: https://chromium-review.googlesource.com/1158313
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580045}

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

@@ -47,17 +47,18 @@ void DesktopLinuxBrowserFrameViewLayout::LayoutNewStyleAvatar(
       new_avatar_button_->GetPreferredSize(),
       delegate_->GetTopAreaHeight() - TitlebarTopThickness(), &button_size,
       &button_spacing);
-  int extra_offset = has_trailing_buttons()
-                         ? nav_button_provider_->GetInterNavButtonSpacing()
-                         : 0;
+  const int extra_offset =
+      has_trailing_buttons() ? nav_button_provider_->GetInterNavButtonSpacing()
+                             : 0;
 
-  int total_width = button_size.width() + button_spacing.right() + extra_offset;
+  const int total_width =
+      button_size.width() + button_spacing.right() + extra_offset;
 
-  int button_x = host->width() - trailing_button_start_ - total_width;
-  int button_y = button_spacing.top() + TitlebarTopThickness();
+  const int button_x = available_space_trailing_x_ - total_width;
+  const int button_y = button_spacing.top() + TitlebarTopThickness();
 
   minimum_size_for_buttons_ += total_width;
-  trailing_button_start_ += total_width;
+  available_space_trailing_x_ -= total_width;
 
   new_avatar_button_->SetBounds(button_x, button_y, button_size.width(),
                                 button_size.height());

chrome/browser/ui/views/frame/opaque_browser_frame_view_layout.h

@@ -49,9 +49,8 @@ class OpaqueBrowserFrameViewLayout : public views::LayoutManager {
       const std::vector<views::FrameButton>& leading_buttons,
       const std::vector<views::FrameButton>& trailing_buttons);
 
-  gfx::Rect GetBoundsForTabStrip(
-      const gfx::Size& tabstrip_preferred_size,
-      int available_width) const;
+  gfx::Rect GetBoundsForTabStrip(const gfx::Size& tabstrip_preferred_size,
+                                 int total_width) const;
 
   gfx::Size GetMinimumSize(int available_width) const;
 
@@ -165,10 +164,10 @@ class OpaqueBrowserFrameViewLayout : public views::LayoutManager {
 
   views::View* new_avatar_button_;
 
-  // How far from the leading/trailing edge of the view the next window control
-  // should be placed.
-  int leading_button_start_;
-  int trailing_button_start_;
+  // The leading and trailing x positions of the empty space available for
+  // laying out titlebar elements.
+  int available_space_leading_x_;
+  int available_space_trailing_x_;
 
   // The size of the window buttons, and the avatar menu item (if any). This
   // does not count labels or other elements that should be counted in a