Allow $TMPDIR to set temporary directory on OS X.

base/files/file_util_mac.mm

Index: base/files/file_util_mac.mm
diff --git a/base/files/file_util_mac.mm b/base/files/file_util_mac.mm
index e9c6c651593571ca425c1a012db8ec76722938ff..8c97aec9803fc98aac8a448a89dae5876b3f4909 100644
--- a/base/files/file_util_mac.mm
+++ b/base/files/file_util_mac.mm
@@ -23,7 +23,13 @@ bool CopyFile(const FilePath& from_path, const FilePath& to_path) {
 }
 
 bool GetTempDir(base::FilePath* path) {
-  NSString* tmp = NSTemporaryDirectory();
+  // In order to facilitate hermetic runs on OS X, first check $TMPDIR
+  NSProcessInfo* procInfo = [NSProcessInfo processInfo];
+  NSString* tmp = [[procInfo environment] objectForKey: @"TMPDIR"];
+  if (tmp == nil) {
+    // Then fallback to the immutable NSTemporaryDirectory value.
+    tmp = NSTemporaryDirectory();
+  }
   if (tmp == nil)
     return false;
   *path = base::mac::NSStringToFilePath(tmp);