Provide a way to suppress implicit MIDL generation rules for actions

For IDL files, GYP ninja normally generates rules to build MIDL
unless there are explicit rules. This special casing only works
for rules, but not actions.
In blink, we are going to add an action which runs blink's IDL compiler
to generate IDL dictionary implementation .h/.cpp files, which means
that we need a way to similar logic for actions.
This CL introduces 'explicit_idl_action' flag for such purpose.

Patch from bashi@chromium.org.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/440293002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1964 78cadc50-ecff-11dd-a971-7dbc132099af