Initialize PlatformEventSource when creating first WebContentsViewEfl.

Initialize PlatformEventSource when creating first WebContentsViewEfl.

In M42 we need to make sure PlatformEventSource object is initialized
somewhere. Aura port does it in aura::Env class. Since we're not using
aura in EFL port we have to do it ourselves.

WebContentsViewEfl is used by all the apps using chromium. Making sure
PlatformEventSource is initialized when the first instance of the class
is crated satisfies chromium requirements and also allows us to avoid
modifying chromium code.

Change-Id: I4dd6087a1b0fc0515023027cfb203b8817f1ce31
Signed-off-by: Piotr Tworek <p.tworek@samsung.com>