Move content-side Feature Policy code into content/{common,child}/feature_policy

content/common/DEPS

 include_rules = [
   "-storage/browser",
 
   "+components/discardable_memory/common",
   "+device/base/synchronization",
   "+services/service_manager/public/cpp",
   "+services/video_capture/public/interfaces",
 
   # No inclusion of WebKit from the browser, other than strictly enum/POD,
   # header-only types, and some selected common code.
   "-third_party/WebKit",
   "+third_party/WebKit/public/platform/WebAddressSpace.h",
   "+third_party/WebKit/public/platform/WebCoalescedInputEvent.h",
   "+third_party/WebKit/public/platform/WebCString.h",
   "+third_party/WebKit/public/platform/WebDisplayMode.h",
   "+third_party/WebKit/public/platform/WebDragOperation.h",
+  "+third_party/WebKit/public/platform/WebFeaturePolicy.h",
   "+third_party/WebKit/public/platform/WebFloatPoint.h",
   "+third_party/WebKit/public/platform/WebFloatRect.h",
   "+third_party/WebKit/public/platform/WebFocusType.h",
   "+third_party/WebKit/public/platform/WebGamepad.h",
   "+third_party/WebKit/public/platform/WebGamepads.h",
   "+third_party/WebKit/public/platform/WebGestureEvent.h",
   "+third_party/WebKit/public/platform/WebHTTPBody.h",
   "+third_party/WebKit/public/platform/WebHistoryScrollRestorationType.h",
   "+third_party/WebKit/public/platform/WebInputEvent.h",
   "+third_party/WebKit/public/platform/WebInsecureRequestPolicy.h",
@@ skipping 43 matching lines @@
   "+third_party/WebKit/public/web/WebPopupType.h",
   "+third_party/WebKit/public/web/WebSandboxFlags.h",
   "+third_party/WebKit/public/web/WebSharedWorkerCreationContextType.h",
   "+third_party/WebKit/public/web/WebSharedWorkerCreationErrors.h",
   "+third_party/WebKit/public/web/WebTextDirection.h",
   "+third_party/WebKit/public/web/WebTreeScopeType.h",
   "+third_party/WebKit/public/web/WebWindowFeatures.h",
   "+third_party/WebKit/public/web/mac/WebScrollbarTheme.h",
   "+third_party/WebKit/public/web/win/WebFontRendering.h"
 ]