Add support for HEAD request mime type getting.

lib/player.js

@@ -1209,7 +1209,9 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
 
       this.assetUri_ = assetUri;
 
-      if (this.shouldUseSrcEquals_(assetUri, mimeType)) {
+      const shouldUseSrcEquals =
+          await this.shouldUseSrcEquals_(assetUri, mimeType);
+      if (shouldUseSrcEquals) {
         await this.initializeSrcEqualsDrmInner_(mimeType);
         await this.srcEqualsInner_(startTime, startTimeOfLoad);
       } else {
@@ -1232,12 +1234,12 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
    *
    * @param {string} assetUri
    * @param {string=} mimeType
-   * @return {boolean}
+   * @return {!Promise.<boolean>}
    *    |true| if the content should be loaded with src=, |false| if the content
    *    should be loaded with MediaSource.
    * @private
    */
-  shouldUseSrcEquals_(assetUri, mimeType) {
+  async shouldUseSrcEquals_(assetUri, mimeType) {
     const Platform = shaka.util.Platform;
     const MimeUtils = shaka.util.MimeUtils;
 
@@ -1255,11 +1257,15 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
       mimeType = shaka.Player.SRC_EQUAL_EXTENSIONS_TO_MIME_TYPES_[extension];
     }
 
-    // TODO: The load graph system has a design limitation that requires routing
-    // destination to be chosen synchronously.  This means we can only make the
-    // right choice about src= consistently if we have a well-known file
-    // extension or API-provided MIME type.  Detection of MIME type from a HEAD
-    // request (as is done for manifest types) can't be done yet.
+    // If no MIME type is provided, and we can't base it on extension, make a
+    // HEAD request to determine it.
+    if (!mimeType) {
+      // TODO: Modify |HlsParser.guessMimeType_| to also use
+      // |shaka.media.ManifestParser.getMimeType|, for standardization?
+      const retryParams = this.config_.manifest.retryParameters;
+      mimeType = await shaka.media.ManifestParser.getMimeType(
+          assetUri, this.networkingEngine_, retryParams);
+    }
 
     if (mimeType) {
       // If we have a MIME type, check if the browser can play it natively.