[ FeaturePolicy ] downloads-without-user-activation

The CL implements a new feature policy hidden behind the flag
FeaturePolicyForSandbox. The policy is the feature policy counterpart to
the sandbox flag 'allow-downloads-without-user-activation'. This policy
is used to help migrate sandbox flags implementation on top of feature
policies.

PA: w3c/webappsec-permissions-policy#289

Bug: 812381
Change-Id: I234bb0486313ab350827315843db22525c2ceaac

feature-policy/feature-policy-for-sandbox/resources/helper.js

@@ -7,8 +7,8 @@ const ignore_features_for_auxilary_context = ["popups", "scripts"];
 
 // Feature-policies that represent specific sandbox flags.
 const sandbox_features = [
-    "forms", "modals", "orientation-lock", "pointer-lock", "popups",
-    "presentation", "scripts", "top-navigation"];
+    "downloads-without-user-activation", "forms", "modals", "orientation-lock",
+    "pointer-lock", "popups", "presentation", "scripts", "top-navigation"];
 
 // TODO(ekaramad): Figure out different inheritance requirements for different
 // policies.