Migrate DeprecatedLower to LowerASCII where only compared with ASCII

This CL changes DeprecatedLower calls to LowerASCII where the result
is only ever used in a comparison against one or more ASCII constants
(including predicates like StartsWith) but is otherwise dead, and I
believe it’s the last CL needed to finish our intent to remove Unicode
case-insensitivity for ASCII constants [1].

The correctness of the image_encoder_utils.cc change isn’t obvious,
but you can verify it by heading to the line in Chromium Code Search
[2], hovering over lowercase_mime_type, and observing that the only
reads are == MIME type literals and by two functions:

• IsSupportedImageMIMETypeForEncoding (line 78), which does the same
• ParseImageEncodingMimeType (line 80), which does the same

These changes are potentially author-facing, except where the constant
in question contains none of {s,k,S,K} [3], but I’ve optimistically
made them without any additional tests based on Rick Byers’ reasoning
on blink-dev [4]. Please let me know if that’s not sufficient here.

[1] https://groups.google.com/a/chromium.org/d/topic/blink-dev/sFOpNuQ91UU
[2] https://source.chromium.org/chromium/chromium/src/+/f7455871a37e53362183771af7de36844df73a38:third_party/blink/renderer/platform/image-encoders/image_encoder_utils.cc;l=36
[3] https://www.unicode.org/Public/UCD/latest/ucd/CaseFolding.txt
[4] https://groups.google.com/a/chromium.org/d/msg/blink-dev/sFOpNuQ91UU/3u1HxbnQCQAJ

Change-Id: I10e621bb2a6947bc3004d9a0c156dcae2e05a6e8
Bug: 627682
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2114510
Commit-Queue: Delan Azabani <dazabani@igalia.com>
Reviewed-by: Frédéric Wang <fwang@igalia.com>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753581}

third_party/blink/renderer/core/clipboard/data_transfer.cc

@@ -204,11 +204,11 @@ static String ConvertDragOperationToEffectAllowed(DragOperation op) {
 }
 
 // We provide the IE clipboard types (URL and Text), and the clipboard types
-// specified in the WHATWG Web Applications 1.0 draft see
-// http://www.whatwg.org/specs/web-apps/current-work/ Section 6.3.5.3
+// specified in the HTML spec. See
+// https://html.spec.whatwg.org/multipage/dnd.html#the-datatransfer-interface
 static String NormalizeType(const String& type,
                             bool* convert_to_url = nullptr) {
-  String clean_type = type.StripWhiteSpace().DeprecatedLower();
+  String clean_type = type.StripWhiteSpace().LowerASCII();
   if (clean_type == kMimeTypeText ||
       clean_type.StartsWith(kMimeTypeTextPlainEtc))
     return kMimeTypeTextPlain;

third_party/blink/renderer/core/editing/commands/editing_command_test.cc

@@ -62,7 +62,7 @@ TEST_F(EditingCommandTest, CreateCommandFromStringCaseFolding) {
   Editor& dummy_editor = GetDocument().GetFrame()->GetEditor();
   for (const auto& entry : kCommandNameEntries) {
     const EditorCommand lower_name_command =
-        dummy_editor.CreateCommand(String(entry.name).DeprecatedLower());
+        dummy_editor.CreateCommand(String(entry.name).LowerASCII());
     EXPECT_EQ(static_cast<int>(entry.type), lower_name_command.IdForHistogram())
         << entry.name;
     const EditorCommand upper_name_command =

third_party/blink/renderer/core/inspector/dom_patch_support.cc

@@ -132,7 +132,7 @@ Node* DOMPatchSupport::PatchNode(Node* node,
     old_list.push_back(CreateDigest(child, nullptr));
 
   // Compose the new list.
-  String markup_copy = markup.DeprecatedLower();
+  String markup_copy = markup.LowerASCII();
   HeapVector<Member<Digest>> new_list;
   for (Node* child = parent_node->firstChild(); child != node;
        child = child->nextSibling())

third_party/blink/renderer/platform/image-encoders/image_encoder_utils.cc

@@ -33,7 +33,7 @@ enum RequestedImageMimeType {
 ImageEncodingMimeType ImageEncoderUtils::ToEncodingMimeType(
     const String& mime_type_name,
     const EncodeReason encode_reason) {
-  String lowercase_mime_type = mime_type_name.DeprecatedLower();
+  String lowercase_mime_type = mime_type_name.LowerASCII();
 
   RequestedImageMimeType requested_mime_type;
   if (mime_type_name.IsNull())

third_party/blink/renderer/platform/mhtml/mhtml_parser.cc

@@ -234,7 +234,7 @@ MIMEHeader* MIMEHeader::ParseHeader(SharedBufferChunkReader* buffer) {
 
 MIMEHeader::Encoding MIMEHeader::ParseContentTransferEncoding(
     const String& text) {
-  String encoding = text.StripWhiteSpace().DeprecatedLower();
+  String encoding = text.StripWhiteSpace().LowerASCII();
   if (encoding == "base64")
     return Encoding::kBase64;
   if (encoding == "quoted-printable")

third_party/blink/renderer/platform/network/mime/mime_type_registry.cc

@@ -193,7 +193,7 @@ bool MIMETypeRegistry::IsSupportedFontMIMEType(const String& mime_type) {
   static const unsigned kFontLen = 5;
   if (!mime_type.StartsWithIgnoringASCIICase("font/"))
     return false;
-  String sub_type = mime_type.Substring(kFontLen).DeprecatedLower();
+  String sub_type = mime_type.Substring(kFontLen).LowerASCII();
   return sub_type == "woff" || sub_type == "woff2" || sub_type == "otf" ||
          sub_type == "ttf" || sub_type == "sfnt";
 }