Simplify assert_throws in two places where error messages were passed

This was discovered in
https://chromium-review.googlesource.com/c/chromium/src/+/980757 by
letting assert_throws also check that the message matches. That change
itself isn't a good idea, but it's still just as well to use plain
TypeErrors here to not make it seems like more is being tested than in
fact is.

Change-Id: I8f964e9b25cc94942cb86f3ee0017530f230e04a
Reviewed-on: https://chromium-review.googlesource.com/995192Reviewed-by: Adam Rice <ricea@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548060}

third_party/WebKit/LayoutTests/external/wpt/fetch/api/abort/general.any.js

@@ -57,7 +57,7 @@ for (const { args, testName } of badRequestArgTests) {
       // Add signal to 2nd arg
       args[1] = args[1] || {};
       args[1].signal = controller.signal;
-      await promise_rejects(t, err, fetch(...args));
+      await promise_rejects(t, new TypeError, fetch(...args));
     }
   }, `TypeError from request constructor takes priority - ${testName}`);
 }

third_party/WebKit/LayoutTests/external/wpt/mediasession/setactionhandler.html

@@ -15,9 +15,7 @@ test(function(t) {
 
 test(function(t) {
   assert_throws(
-      new TypeError("Failed to execute 'setActionHandler' on 'MediaSession':" +
-                    "The provided value 'invalid' is not a valid enum value" +
-                    "of type MediaSessionAction."),
+      new TypeError,
       _ => { window.navigator.mediaSession.setActionHandler("invalid", null); });
 }, "Test that setActionHandler() throws exception for unsupported actions");