Add non-member non-mutating methods for common gfx::Rect operations.

content/browser/renderer_host/render_widget_host_view_aura.cc

Index: content/browser/renderer_host/render_widget_host_view_aura.cc
diff --git a/content/browser/renderer_host/render_widget_host_view_aura.cc b/content/browser/renderer_host/render_widget_host_view_aura.cc
index 2b7bff8a079c9060a367804516672b795b754884..fa2b8aca180dde2780c5eb8c73840088f980aa2c 100644
--- a/content/browser/renderer_host/render_widget_host_view_aura.cc
+++ b/content/browser/renderer_host/render_widget_host_view_aura.cc
@@ -557,8 +557,7 @@ void RenderWidgetHostViewAura::DidUpdateBackingStore(
     SchedulePaintIfNotInClip(scroll_rect, clip_rect);
 
   for (size_t i = 0; i < copy_rects.size(); ++i) {
-    gfx::Rect rect = copy_rects[i];
-    rect.Subtract(scroll_rect);
+    gfx::Rect rect = gfx::Subtraction(copy_rects[i], scroll_rect);
     if (rect.IsEmpty())
       continue;
 
@@ -1168,8 +1167,7 @@ gfx::Rect RenderWidgetHostViewAura::ConvertRectToScreen(const gfx::Rect& rect) {
 }
 
 gfx::Rect RenderWidgetHostViewAura::GetCaretBounds() {
-  gfx::Rect rect = selection_start_rect_;
-  rect.Union(selection_end_rect_);
+  const gfx::Rect rect = gfx::Union(selection_start_rect_, selection_end_rect_);
   return ConvertRectToScreen(rect);
 }
 
@@ -1799,8 +1797,7 @@ void RenderWidgetHostViewAura::SchedulePaintIfNotInClip(
     const gfx::Rect& rect,
     const gfx::Rect& clip) {
   if (!clip.IsEmpty()) {
-    gfx::Rect to_paint = rect;
-    to_paint.Subtract(clip);
+    gfx::Rect to_paint = gfx::Subtraction(rect, clip);
     if (!to_paint.IsEmpty())
       window_->SchedulePaintInRect(to_paint);
   } else {