blink/html: convert an enum constant to a constexpr

This constant is sometimes implicitly converted to a floating-point
number, which clang complains about, since such conversions are often
bugs.

Using a `constexpr int` should be equivalent.

Bug: 1058218
Change-Id: Ic6cafe54b97053b841caaa094a400727eb4cebb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2096097Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748893}

third_party/blink/renderer/core/html/parser/html_srcset_parser.h

@@ -41,7 +41,7 @@ namespace blink {
 
 class Document;
 
-enum { kUninitializedDescriptor = -1 };
+constexpr int kUninitializedDescriptor = -1;
 
 class DescriptorParsingResult {
   STACK_ALLOCATED();