Use native weak references instead of our wrapper.

Use native weak references instead of our wrapper.

Revert "Add check null of getRealObject(). When a WebView is"
This reverts commit b47ea2b1184167767692f88c1e5f3e4e89db04b2.

Revert "Update the comment for checking null in needtouchevents."
This reverts commit 1e67e45d20ead40b0407618abbc5e20840dc3c80.

Revert "Check null for WebViewCore's nativeClass before calling"
This reverts commit 992afbebfb735ec2dd52b4166c22ab55827d88ed.

Fix a few places where DeleteLocalRef and DeleteGlobalRef were used incorrectly.
Fix a compilation warning as well.