Apply [SecureContext] to StorageManager

The [SecureContext] extended attribute now applies correctly to
interfaces, so make sure that the StorageManager interface is not
exposed in non-secure contexts, not just the `navigator.storage` entry
point.

Bug: 726510
Change-Id: Ie141aa4e29e261f1107af292f74f8063370c3748
Reviewed-on: https://chromium-review.googlesource.com/802676Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521056}

third_party/WebKit/Source/modules/quota/StorageManager.idl

@@ -4,8 +4,8 @@
 
 // https://storage.spec.whatwg.org/#storagemanager
 
-// TODO(jsbell): Should have [SecureContext] on interface
 [
+    SecureContext,
     Exposed=(Window,Worker)
 ] interface StorageManager {
     [CallWith=ScriptState, MeasureAs=DurableStoragePersisted] Promise<boolean> persisted();