Use forward declaration instead of include for NPObject

All we use is just a NPObject* here, there is no need to include
the entire third_party/npapi/bindings/npruntime.h file, plus that
file includes third_party/npapi/bindings/npapi.h which further
includes system gl.h in OS X, which contains a GLES2TexImage2D
definition that conflicts with the definition in
gpu/GLES2/gl2chromium_autogen.h. Similar issues were fixed in CL
https://codereview.chromium.org/11794019/ We will hit this issue
when trying to include V8NPObject.h along with ChromeClientImpl.h.

Review URL: https://codereview.chromium.org/289273003

git-svn-id: svn://svn.chromium.org/blink/trunk@174283 bbb929c8-8fbe-4397-9dbb-9b2b20218538