Generator,BUILD.gn: add optional python path, use it for Jinja2

This makes the Jinja2 (and MarkupSafe) installation hermetic by
adding it to the DEPS and making the code generator add them in the
first spot of the python path.

scripts/dawn_overrides_with_defaults.gni

@@ -43,3 +43,7 @@ if (!defined(dawn_shaderc_dir)) {
 if (!defined(dawn_glfw_dir)) {
   dawn_glfw_dir = "//third_party/glfw"
 }
+
+if (!defined(dawn_jinja2_dir)) {
+  dawn_jinja2_dir = "//third_party/jinja2"
+}