[Fetch] Always use utf-8 for decoding in text()

third_party/WebKit/LayoutTests/http/tests/fetch/script-tests/body-mixin.js

 if (self.importScripts) {
   importScripts('../resources/fetch-test-helpers.js');
 }
 
 function readStream(reader, values) {
   reader.read().then(function(r) {
       if (!r.done) {
         values.push(r.value);
         readStream(reader, values);
       }
@@ skipping 156 matching lines @@
           assert_false(response.bodyUsed);
           var p = response.text();
           assert_true(response.bodyUsed);
           return p;
         })
       .then(function(text) {
           assert_equals(text, '\u4e2d\u6587 Gem\u00fcse\n');
         })
     }, 'NonAsciiTextTest');
 
+promise_test(function(test) {
+    return fetch('/fetch/resources/bom-utf-8.php')
+      .then(function(response) { return response.text(); })
+      .then(function(text) {
+          assert_equals(text, '\u4e09\u6751\u304b\u306a\u5b50',
+                        'utf-8 string with BOM is decoded as utf-8 and ' +
+                        'BOM is not included in the decoded result.');
+        })
+    }, 'BOMUTF8Test');
+
+promise_test(function(test) {
+    return fetch('/fetch/resources/bom-utf-16le.php')
+      .then(function(response) { return response.text(); })
+      .then(function(text) {
+          assert_equals(text, '\ufffd\ufffd\tNQgK0j0P[',
+                        'utf-16le string is decoded as if utf-8 ' +
+                        'even if the data has utf-16le BOM.');
+        })
+    }, 'BOMUTF16LETest');
+
+promise_test(function(test) {
+    return fetch('/fetch/resources/bom-utf-16be.php')
+      .then(function(response) { return response.text(); })
+      .then(function(text) {
+          assert_equals(text, '\ufffd\ufffdN\tgQ0K0j[P',
+                        'utf-16be string is decoded as if utf-8 ' +
+                        'even if the data has utf-16be BOM.');
+        })
+    }, 'BOMUTF16BETest');
+
 test(t => {
     var req = new Request('/');
     assert_false(req.bodyUsed);
     req.text();
     assert_false(req.bodyUsed);
   }, 'BodyUsedShouldNotBeSetForNullBody');
 
 test(t => {
     var req = new Request('/', {method: 'POST', body: ''});
     assert_false(req.bodyUsed);
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 promise_test(t => {
     var res = new Response('');
     res.body.getReader();
     return res.text().then(unreached_fulfillment(t), e => {
         assert_equals(e.name, 'TypeError');
       });
   }, 'Locked => text');
 
 done();