Change snapshot operation for fixing blink issue

Change snapshot operation for fixing blink issue

[Issue] N/A
[Problem] When Pd is opened, its box is blinked for a while
[Cause] To show PD slide effect, Home requests its window to dali engine.
        even if web-provider doesn't prepare valid frame on render buffers for web dbox,
        dali graphic engine updates Homescreen's window using the buffer of web dbox.
        So sometimes, buffer for web dbox is showed as blank on Homescreen's window for a while.
[Solution] web-provider keep last snapshot of each web dbox by finishing to load box page.

Change-Id: Idb1d9d5d23270729c222e7226c1abba0dafb1f94