Bug 1399100, temporarily expose an NSPR helper function, only for Fir…

…efox 57, feedback+=mcmanus, r=fkiefer

This is an exception from the usual NSPR release upgrade process and essentially an agreed temporary fork, which will be undone for Firefox 58.
This change isn't causing any harm, because the temporary API is used only for Windows, and only accessed using dynamic symbol lookup, so it will not cause any issue on operating systems that use a system-wide install copy of NSPR without the symbol.
UPGRADE_NSPR_RELEASE

nsprpub/patches/README.TXT

@@ -0,0 +1,9 @@
+bug1399100-temporary.patch :
+  This is a temporary patch as described in
+    https://bugzilla.mozilla.org/show_bug.cgi?id=1399100
+  It is temporarily necessary because of the changes in
+    https://bugzilla.mozilla.org/show_bug.cgi?id=1384633
+  The patch should be re-applied during the Firefox 57 release cycle,
+  if other changes to NSPR are necessary during the lifetime of Firefox 57.
+  The temporary API will be removed for Firefox 58, as agreed in
+    https://bugzilla.mozilla.org/show_bug.cgi?id=1394808

nsprpub/patches/bug1399100-temporary.patch

@@ -0,0 +1,16 @@
+diff --git a/nsprpub/pr/src/nspr.def b/nsprpub/pr/src/nspr.def
+--- a/nsprpub/pr/src/nspr.def
++++ b/nsprpub/pr/src/nspr.def
+@@ -457,8 +457,12 @@ EXPORTS ;-
+ ;+} NSPR_4.9.2;
+ ;+# Function PR_DuplicateEnvironment had been added in NSPR 4.10.9,
+ ;+# but we neglected to add it to nspr.def until NSPR 4.12
+ ;+NSPR_4.12 {
+ ;+      global:
+ 		PR_DuplicateEnvironment;
+ 		PR_GetEnvSecure;
+ ;+} NSPR_4.10.3;
++;+NSPR_4.17_fork {
++;+      global:
++		PR_EXPERIMENTAL_ONLY_IN_4_17_GetOverlappedIOHandle;
++;+} NSPR_4.16;

nsprpub/pr/src/nspr.def

@@ -462,3 +462,7 @@ EXPORTS ;-
 		PR_DuplicateEnvironment;
 		PR_GetEnvSecure;
 ;+} NSPR_4.10.3;
+;+NSPR_4.17_fork {
+;+      global:
+		PR_EXPERIMENTAL_ONLY_IN_4_17_GetOverlappedIOHandle;
+;+} NSPR_4.16;