Switching std::string::npos to WTF::kNotFound for string search

I made a change earlier today not realizing that the strings used in
this class were WTF::Strings and not std::strings.  This is a follow-up
CL to address feedback that I should be using WTF::kNotFound instead of

std: :string::npos for the not found case.
Change-Id: I6f26ca2b9699e8a085a5368236cb54731480e9d7
Reviewed-on: https://chromium-review.googlesource.com/987536
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547163}

third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp

@@ -428,11 +428,11 @@ bool FingerprintMismatch(String old_sdp, String new_sdp) {
   // Look for fingerprint being modified. Not allowed.  Handle differences in
   // line endings ('\r\n' vs, '\n' when looking for the end of the fingerprint).
   size_t old_fingerprint_end = old_sdp.Find("\r\n", old_fingerprint_pos + 1);
-  if (old_fingerprint_end == std::string::npos) {
+  if (old_fingerprint_end == WTF::kNotFound) {
     old_fingerprint_end = old_sdp.Find("\n", old_fingerprint_pos + 1);
   }
   size_t new_fingerprint_end = new_sdp.Find("\r\n", new_fingerprint_pos + 1);
-  if (new_fingerprint_end == std::string::npos) {
+  if (new_fingerprint_end == WTF::kNotFound) {
     new_fingerprint_end = new_sdp.Find("\n", new_fingerprint_pos + 1);
   }
   return old_sdp.Substring(old_fingerprint_pos,