Eliminate DeprecatedLower in ResourceResponse::HttpContentType()

This patch changes ResourceResponse::HttpContentType() to use
LowerASCII instead of DeprecatedLower.

DeprecatedLower has partial Unicode conversions, such as
converting U+212A to 'k', but lowering only ASCII should be
sufficient for most web platform usages.

For MIME type, [parse a MIME type] uses [ASCII lowercase].

Currently, there are 6 usages of AtomicString::DeprecatedLower
and 37 usages of String::DeprecatedLower. This patch reduces
one of the 6 usages.

[ASCII lowercase]: https://infra.spec.whatwg.org/#ascii-lowercase
[parse a MIME type]: https://mimesniff.spec.whatwg.org/#parse-a-mime-type

Bug: 627682
Change-Id: I9f44fbe1583618c3f1250af8e417dedf6cd41563
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720994
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682127}

third_party/blink/renderer/platform/loader/fetch/resource_response.cc

@@ -376,7 +376,7 @@ bool ResourceResponse::IsAttachment() const {
 
 AtomicString ResourceResponse::HttpContentType() const {
   return ExtractMIMETypeFromMediaType(
-      HttpHeaderField(http_names::kContentType).DeprecatedLower());
+      HttpHeaderField(http_names::kContentType).LowerASCII());
 }
 
 bool ResourceResponse::WasCached() const {