perf: Optimization to resolve uris (#5657)

Chained use of maps and reduce on legacy devices reduces performance. So
if there is only one url to generate we can optimize performance.

lib/util/manifest_parser_utils.js

@@ -29,13 +29,19 @@ shaka.util.ManifestParserUtils = class {
       return baseUris;
     }
 
+    if (baseUris.length == 1 && relativeUris.length == 1) {
+      const baseUri = new goog.Uri(baseUris[0]);
+      const relativeUri = new goog.Uri(relativeUris[0]);
+      return [baseUri.resolve(relativeUri).toString()];
+    }
+
     const relativeAsGoog = relativeUris.map((uri) => new goog.Uri(uri));
     // Resolve each URI relative to each base URI, creating an Array of Arrays.
     // Then flatten the Arrays into a single Array.
-    return baseUris.map((uri) => new goog.Uri(uri))
-        .map((base) => relativeAsGoog.map((i) => base.resolve(i)))
-        .reduce(Functional.collapseArrays, [])
-        .map((uri) => uri.toString());
+    return baseUris.map((uri) => {
+      const base = new goog.Uri(uri);
+      return relativeAsGoog.map((i) => base.resolve(i).toString());
+    }).reduce(Functional.collapseArrays, []);
   }