Remove assert_throws usage in http/tests/mojo/

'assert_throws' has been deprecated in WPT, replaced by JS
and DOM-specific variants. http/tests/mojo should use the
JS variant, but that requires mojo.test.InputError to be a
real Error subclass.

Bug: 1051932
Change-Id: Icdc16cc69a856f3cfb65c0e6686f91d5bcf0eae1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125949Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754967}

third_party/blink/web_tests/http/tests/mojo/resources/validation_test_input_parser.js

@@ -10,18 +10,21 @@
   // Files and Lines represent the raw text from an input string
   // or ".data" file.
 
-  function InputError(message, line) {
-    this.name = "InputError";
-    this.message = message;
-    this.line = line;
-  }
+  class InputError extends Error {
+    constructor(message, line) {
+      super(message);
 
-  InputError.prototype.toString = function() {
-    var s = 'Error: ' + this.message;
-    if (this.line)
-      s += ', at line ' +
-           (this.line.number + 1) + ': "' + this.line.contents + '"';
-    return s;
+      this.name = "InputError";
+      this.line = line;
+    }
+
+    toString() {
+      var s = 'Error: ' + this.message;
+      if (this.line)
+        s += ', at line ' +
+             (this.line.number + 1) + ': "' + this.line.contents + '"';
+      return s;
+    }
   }
 
   function File(contents) {

third_party/blink/web_tests/http/tests/mojo/validation.html

@@ -100,7 +100,7 @@ test(() => {
 
 test(() => {
   function parserShouldFail(input) {
-    assert_throws(new mojo.test.InputError(), function() {
+    assert_throws_js(mojo.test.InputError, function() {
       mojo.test.parseTestMessage(input);
     });
   }