fixup! Implement "certificate,pem,set" callback.

fixup! Implement "certificate,pem,set" callback.

ContentBrowserClient::SetCertificatePem is called by chromium on IO
thread. We can't invoke EWK certificate,pem,set callback from non UI
threads. EWK is not a thread safe API. Also the code was triggering
asserts in chromium on every single https site I could find.

Bug: http://web.sec.samsung.net/bugzilla/show_bug.cgi?id=11023
Reviewed by: Antonio Gomes, Kamil Klimek, SeungSeop Park

Change-Id: I121cbda6f9aca3e3a0749bb0b62981cb5dfc5952
Signed-off-by: Piotr Tworek <p.tworek@samsung.com>