Increment MatchResult::current_tree_order_ by one for each tree scope

This is effectively what we're doing already, since the size of the
author_range_ends_ vector just increased by one (and the size of that
vector never decreases).

Setting the current_tree_order_ when finishing user rules is not
necessary. I don't know why we did this. Possibly it's a remnant of
a previous solution.

Overall, what matters is that current_tree_order_ is monotonically
increasing for each call to FinishAddingAuthorRulesForTreeScope.

This prepares the ua/user/author ranges for removal in a subsequent CL.

Change-Id: I99947e5bfdf727ba772d296ecab65b424625dd91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2310495
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791167}

third_party/blink/renderer/core/css/resolver/match_result.cc

@@ -33,6 +33,7 @@
 #include <memory>
 #include <type_traits>
 
+#include "base/numerics/clamped_math.h"
 #include "third_party/blink/renderer/core/css/css_property_value_set.h"
 #include "third_party/blink/renderer/core/css/style_rule.h"
 #include "third_party/blink/renderer/platform/wtf/math_extras.h"
@@ -79,11 +80,11 @@ void MatchResult::FinishAddingUserRules() {
       user_range_ends_.back() == matched_properties_.size())
     return;
   user_range_ends_.push_back(matched_properties_.size());
-  current_tree_order_ = clampTo<uint16_t>(user_range_ends_.size());
 }
 
 void MatchResult::FinishAddingAuthorRulesForTreeScope() {
   DCHECK_EQ(current_origin_, CascadeOrigin::kAuthor);
+  current_tree_order_ = base::ClampAdd(current_tree_order_, 1);
   // Don't add empty ranges.
   if (author_range_ends_.IsEmpty() && user_range_ends_.IsEmpty() &&
       ua_range_end_ == matched_properties_.size())
@@ -95,7 +96,6 @@ void MatchResult::FinishAddingAuthorRulesForTreeScope() {
       author_range_ends_.back() == matched_properties_.size())
     return;
   author_range_ends_.push_back(matched_properties_.size());
-  current_tree_order_ = clampTo<uint16_t>(author_range_ends_.size());
 }
 
 MatchedExpansionsRange MatchResult::Expansions(const Document& document,

third_party/blink/renderer/core/css/resolver/match_result.h

@@ -59,10 +59,6 @@ struct CORE_EXPORT MatchedProperties {
     // not used at all for the UA origin. Hence, it is not possible to compare
     // tree_orders from two different origins.
     //
-    // Note also that the tree_order will start at ~0u and then decrease.
-    // This is because we currently store the matched properties in reverse
-    // order.
-    //
     // https://drafts.csswg.org/css-scoping/#shadow-cascading
     uint16_t tree_order;
   };