WTF: Add comments and tests for Unicode aware case-insensitive string operations.

third_party/WebKit/Source/wtf/text/WTFString.h

Index: third_party/WebKit/Source/wtf/text/WTFString.h
diff --git a/third_party/WebKit/Source/wtf/text/WTFString.h b/third_party/WebKit/Source/wtf/text/WTFString.h
index 3c516adf647f803ba4eb9509c21688d456bb787d..c2b55e4b312ec04c18ca580a3b9381824bf0a2ac 100644
--- a/third_party/WebKit/Source/wtf/text/WTFString.h
+++ b/third_party/WebKit/Source/wtf/text/WTFString.h
@@ -187,7 +187,9 @@ class WTF_EXPORT String {
                : kNotFound;
   }
 
-  // Unicode aware case insensitive string matching.
+  // Unicode aware case insensitive string matching. Non-ASCII characters might
+  // match to ASCII characters. This function is rarely used to implement web
+  // platform features.
   size_t findIgnoringCase(const StringView& value, unsigned start = 0) const {
     return m_impl ? m_impl->findIgnoringCase(value, start) : kNotFound;
   }
@@ -280,7 +282,10 @@ class WTF_EXPORT String {
   String left(unsigned len) const { return substring(0, len); }
   String right(unsigned len) const { return substring(length() - len, len); }
 
-  // Returns a lowercase/uppercase version of the string
+  // Returns a lowercase/uppercase version of the string. These functions might
+  // convert non-ASCII characters to ASCII characters. For example, lower() for
+  // U+212A is 'k', upper() for U+017F is 'S'.
+  // These functions are rarely used to implement web platform features.
   String lower() const;
   String upper() const;