[M108 Migration][VD]Add NULL pointer checking and logs for EWebView::Hide

[M108 Migration][VD]Add NULL pointer checking and logs for EWebView::Hide

1. The pointer(web_contents_) didn't do NULL check before use.
This may cause coredump if web_contents_ is NULL. So add NULL pointer
checking for function Hide.
2. Add some logs for function Hide and EWebView and ~EWebView to trace
certain WebView instance's life cycle.

Reference:
 - https://review.tizen.org/gerrit/#/c/285660/

Change-Id: I315c2e84f7c2f3260af2ddc77ccfad1b9f7e9232
Signed-off-by: jiangyuwei <yuwei.jiang@samsung.com>