[infra] Make download of jsshell conditional

The jsshell is used rarely for benchmarking and testing.
Make download of it conditional, at least until it is uploaded
and moved to CPID, to help migration to python3.

To download the shell in a checkout, add
"checkout_javascript_engines": True
to the "custom_vars" in a .gclient solution.

Change-Id: I693a4b402ffebca28d94c124624c4bd897315818
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/195261
Reviewed-by: Alexander Thomas <athom@google.com>

DEPS

@@ -54,6 +54,10 @@ vars = {
   # Checkout Android dependencies only on Mac and Linux.
   "download_android_deps": 'host_os == "mac" or host_os == "linux"',
 
+  # Checkout extra javascript engines for testing or benchmarking.
+  # d8, the V8 shell, is always checked out.
+  "checkout_javascript_engines": False,
+
   # As Flutter does, we use Fuchsia's GN and Clang toolchain. These revision
   # should be kept up to date with the revisions pulled by the Flutter engine.
   # The list of revisions for these tools comes from Fuchsia, here:
@@ -662,6 +666,7 @@ hooks = [
       "--directory",
       Var('dart_root') + "/third_party/firefox_jsshell",
     ],
+    "condition": "checkout_javascript_engines",
   },
   {
     # Pull Debian sysroot for i386 Linux