feat: Improve performance of multi-period DASH parsing (#5350)

For manifests with many streams, we saw the bulk of the manifest parse
time going towards period combiner. Specifically: splitting codecs and
comparing DRMs. This change cuts our manifest parse time for large
manifests by nearly 30%.

There are two changes:

1. Memoize the normalized codecs inside the period combiner.
2. Short circuit the DRM compatibility check if the DRMInfos arrays are
the same object. We run a custom manifest parser and are therefore able
to re-use the same drm infos array for functionally equivalent drms, but
short circuiting also helps since in the case that the upstream period
combiner is comparing a candidate stream with itself.

lib/media/drm_engine.js

@@ -2005,6 +2005,10 @@ shaka.media.DrmEngine = class {
       return true;
     }
 
+    if (drms1 === drms2) {
+      return true;
+    }
+
     return shaka.media.DrmEngine.getCommonDrmInfos(
         drms1, drms2).length > 0;
   }

lib/util/periods.js

@@ -1067,8 +1067,13 @@ shaka.util.PeriodCombiner = class {
    * @private
    */
   static areAVStreamsCompatible_(outputStream, candidate) {
-    const getCodec = (codecs) =>
-      shaka.util.MimeUtils.getNormalizedCodec(codecs);
+    const getCodec = (codecs) => {
+      if (!shaka.util.PeriodCombiner.memoizedCodecs.has(codecs)) {
+        const normalizedCodec = shaka.util.MimeUtils.getNormalizedCodec(codecs);
+        shaka.util.PeriodCombiner.memoizedCodecs.set(codecs, normalizedCodec);
+      }
+      return shaka.util.PeriodCombiner.memoizedCodecs.get(codecs);
+    };
     // Check MIME type and codecs, which should always be the same.
     if (candidate.mimeType != outputStream.mimeType ||
         getCodec(candidate.codecs) != getCodec(outputStream.codecs)) {
@@ -1666,3 +1671,8 @@ shaka.util.PeriodCombiner.BetterOrWorse = {
   EQUAL: 0,
   WORSE: -1,
 };
+
+/**
+ * @private {Map<string, string>}
+ */
+shaka.util.PeriodCombiner.memoizedCodecs = new Map();