Eliminate DeprecatedLower in ParseCacheControlDirectives()

This patch changes ParseCacheControlDirectives() 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.

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

Bug: 627682
Change-Id: Ib48c430d7afe3e16be5dc567593b7f4b2634c4e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1721210
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681756}

third_party/blink/renderer/platform/network/http_parsers.cc

@@ -562,7 +562,7 @@ CacheControlHeader ParseCacheControlDirectives(
     // This is deprecated and equivalent to Cache-control: no-cache
     // Don't bother tokenizing the value, it is not important
     cache_control_header.contains_no_cache =
-        pragma_value.DeprecatedLower().Contains(kNoCacheDirective);
+        pragma_value.LowerASCII().Contains(kNoCacheDirective);
   }
   return cache_control_header;
 }