Rename StringCs c_str() to unterminated_c_str().

Since there is no guarantee of termination if the StringC was
extracted from a snippet of another string.  Make it more obvious
that things like

  strlen(str.unterminated_c_str())

might be a bad idea.

Change-Id: I7832248ed89ebbddf5c0bcd402aac7d40ec2adc2
Reviewed-on: https://pdfium-review.googlesource.com/8170
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
diff --git a/testing/xfa_js_embedder_test.cpp b/testing/xfa_js_embedder_test.cpp
index e5a30f6..d36cc81 100644
--- a/testing/xfa_js_embedder_test.cpp
+++ b/testing/xfa_js_embedder_test.cpp
@@ -59,7 +59,7 @@
 
   CFXJSE_Value msg(GetIsolate());
   value_->GetObjectPropertyByIdx(1, &msg);
-  fprintf(stderr, "JS: %.*s\n", input.GetLength(), input.c_str());
+  fprintf(stderr, "JS: %.*s\n", input.GetLength(), input.unterminated_c_str());
   // If the parsing of the input fails, then v8 will not run, so there will be
   // no value here to print.
   if (msg.IsString() && !msg.ToWideString().IsEmpty())