Auto merge of #8454 - jdm:media, r=KiChjang

Implement basic <media> infrastructure

This gets us to the point where we can start playing with actually integrating rust-media to process the data received by the network request, as currently it's just ignored.

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components/script/document_loader.rs

@@ -21,6 +21,7 @@ pub enum LoadType {
     Subframe(Url),
     Stylesheet(Url),
     PageSource(Url),
+    Media(Url),
 }
 
 impl LoadType {
@@ -30,6 +31,7 @@ impl LoadType {
             LoadType::Script(ref url) |
             LoadType::Subframe(ref url) |
             LoadType::Stylesheet(ref url) |
+            LoadType::Media(ref url) |
             LoadType::PageSource(ref url) => url,
         }
     }
@@ -39,7 +41,8 @@ impl LoadType {
             LoadType::Image(_) => LoadContext::Image,
             LoadType::Script(_) => LoadContext::Script,
             LoadType::Subframe(_) | LoadType::PageSource(_) => LoadContext::Browsing,
-            LoadType::Stylesheet(_) => LoadContext::Style
+            LoadType::Stylesheet(_) => LoadContext::Style,
+            LoadType::Media(_) => LoadContext::AudioVideo,
         }
     }
 }