Goma: Support GOMA_DIR env var in GN.

If developer installs goma to location different from $HOME/goma,
GOMA_DIR should be set as per official goma docs.

Change-Id: I35d952a837f6f734ef9d45e415d5a49596f88b58
Reviewed-on: https://chromium-review.googlesource.com/918681
Commit-Queue: Greg Kraynov <kraynov@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536772}

build/toolchain/goma.gni

@@ -17,8 +17,13 @@ declare_args() {
     # Absolute directory containing the gomacc.exe binary.
     goma_dir = "C:\src\goma\goma-win64"
   } else {
-    # Absolute directory containing the gomacc binary.
-    goma_dir = getenv("HOME") + "/goma"
+    if (getenv("GOMA_DIR") != "") {
+      # Absolute directory containing the gomacc binary.
+      goma_dir = getenv("GOMA_DIR")
+    } else {
+      # Absolute directory containing the gomacc binary.
+      goma_dir = getenv("HOME") + "/goma"
+    }
   }
 }