[base/files] Respect MAC_CHROMIUM_TMPDIR instead of TMPDIR on macOS.

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 5a99aa0e81de2e19110872cc168e53b93555d0c5..9cc3d30f8390b728ae72c7f0176e295027621a84 100644
--- a/base/files/file_util_mac.mm
+++ b/base/files/file_util_mac.mm
@@ -24,9 +24,9 @@ bool CopyFile(const FilePath& from_path, const FilePath& to_path) {
 }
 
 bool GetTempDir(base::FilePath* path) {
-  // In order to facilitate hermetic runs on macOS, first check $TMPDIR.
-  // NOTE: $TMPDIR is ALMOST ALWAYS set on macOS (unless the user un-set it).
-  const char* env_tmpdir = getenv("TMPDIR");
+  // In order to facilitate hermetic runs on macOS, first check
+  // $MAC_CHROMIUM_TMPDIR.

[Mark Mentovai, 2017/03/27 20:36:31]

Explain why not TMPDIR here, since it’s a natural question to ask, and our kids
shouldn’t have to spelunk through file history to get the answer.

+  const char* env_tmpdir = getenv("MAC_CHROMIUM_TMPDIR");

[iannucci, 2017/03/25 00:45:04]

I don't have strong feelings about this name.. if someone else does, I'm happy
to change it.

[Mark Mentovai, 2017/03/27 20:36:31]

Maybe [D]CHECK that the length is within a reasonable range here, to avoid
hard-to-diagnose problems like bug 698759.

   if (env_tmpdir) {
     *path = base::FilePath(env_tmpdir);
     return true;