Fix luci-go isolate support on OSX.

The --extra-variable FOO BAR format is not supported by the Go implementation.
Switch to --extra-variable FOO=BAR. Using the second form is only a problem when
the value has spaces and only on Windows and this case is OSX only.

TBR=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1152243003

Cr-Original-Commit-Position: refs/heads/master@{#331218}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ed6da5c8c4695faeb54aa354f48616cfd8207f71

isolate.gypi

@@ -106,10 +106,8 @@
       'conditions': [
         # Note: When gyp merges lists, it appends them to the old value.
         ['OS=="mac"', {
-          # <(mac_product_name) can contain a space, so don't use FOO=<(FOO)
-          # form.
           'action': [
-            '--extra-variable', 'mac_product_name', '<(mac_product_name)',
+            '--extra-variable', 'mac_product_name=<(mac_product_name)',
           ],
         }],
         ["test_isolation_outdir!=''", {