Change Style::resolveColumnGap() return from Length to Style::Length

Summary:
X-link: facebook/yoga#1525

Accidentally left this inconsistent with some of the refactoring. Rename the lone usage of `Length` within Style class to `Style::Length` to match the rest of the code.

This is functionally identical as before.

Changelog: [Internal]

bypass-github-export-checks

Reviewed By: yungsters

Differential Revision: D52096820

fbshipit-source-id: d6c569a02fb27a6e7548a9c12ff764afb823a282

packages/react-native/ReactCommon/yoga/yoga/style/Style.h

@@ -215,7 +215,7 @@ class YG_EXPORT Style {
     aspectRatio_ = value;
   }
 
-  Length resolveColumnGap() const {
+  Style::Length resolveColumnGap() const {
     if (gap_[yoga::to_underlying(Gutter::Column)].isDefined()) {
       return gap_[yoga::to_underlying(Gutter::Column)];
     } else {