Convert ByteString::{Format|FormatV} to static methods

This CL moves the Format and FormatV methods of ByteString to be static.

Bug: pdfium:934
Change-Id: I9c30455a789aff9f619b9d5bf89c0712644f2d9a
Reviewed-on: https://pdfium-review.googlesource.com/18650
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/fpdfattachment.cpp b/fpdfsdk/fpdfattachment.cpp
index 7402114..8b3c8fe 100644
--- a/fpdfsdk/fpdfattachment.cpp
+++ b/fpdfsdk/fpdfattachment.cpp
@@ -232,11 +232,13 @@
   // Set the creation date of the new attachment in the dictionary.
   CFX_DateTime dateTime;
   dateTime.Now();
-  ByteString bsDateTime;
-  bsDateTime.Format("D:%d%02d%02d%02d%02d%02d", dateTime.GetYear(),
-                    dateTime.GetMonth(), dateTime.GetDay(), dateTime.GetHour(),
-                    dateTime.GetMinute(), dateTime.GetSecond());
-  pParamsDict->SetNewFor<CPDF_String>("CreationDate", bsDateTime, false);
+  pParamsDict->SetNewFor<CPDF_String>(
+      "CreationDate",
+      ByteString::Format("D:%d%02d%02d%02d%02d%02d", dateTime.GetYear(),
+                         dateTime.GetMonth(), dateTime.GetDay(),
+                         dateTime.GetHour(), dateTime.GetMinute(),
+                         dateTime.GetSecond()),
+      false);
 
   // Set the checksum of the new attachment in the dictionary.
   pParamsDict->SetNewFor<CPDF_String>(