Disallow implicit conversion in a CSSParserContext constructor

There's a CSSParserContext ctor that allows implicit conversion from a
|const CSSParserContext*| to |const CSSParserContext&|, where the latter
is actually a new object constructed on stack with different member
values.

So this patch stops the conversion.

Change-Id: Ibf98e8b0012122d2fc42e3fc4b4b38e8fe9b797a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037854Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738484}

third_party/blink/renderer/core/css/parser/css_parser_context.h

@@ -37,8 +37,8 @@ class CORE_EXPORT CSSParserContext final
   // FIXME: This constructor shouldn't exist if we properly piped the UseCounter
   // through the CSS subsystem. Currently the UseCounter life time is too crazy
   // and we need a way to override it.
-  CSSParserContext(const CSSParserContext* other,
-                   const Document* use_counter_document = nullptr);
+  explicit CSSParserContext(const CSSParserContext* other,
+                            const Document* use_counter_document = nullptr);
 
   CSSParserContext(const CSSParserContext* other,
                    const KURL& base_url_override,