Use ASCII case folding for encoding type.

Per spec, this should use ASCII, not Unicode, case folding:
  https://html.spec.whatwg.org/#enumerated-attribute

Bug: 627682
Change-Id: I2e11153c3c391fd2a8e720b28d2412da59795c12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575085Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652654}

third_party/blink/renderer/platform/network/encoded_form_data.h

@@ -168,9 +168,9 @@ class PLATFORM_EXPORT EncodedFormData : public RefCounted<EncodedFormData> {
   }
 
   static EncodingType ParseEncodingType(const String& type) {
-    if (DeprecatedEqualIgnoringCase(type, "text/plain"))
+    if (EqualIgnoringASCIICase(type, "text/plain"))
       return kTextPlain;
-    if (DeprecatedEqualIgnoringCase(type, "multipart/form-data"))
+    if (EqualIgnoringASCIICase(type, "multipart/form-data"))
       return kMultipartFormData;
     return kFormURLEncoded;
   }