Fix "uninitialised" m_isVisible value in WebView when WebPageProxy is created.

WebPageProxy calls m_pageClient->isVisible() within its ctor, but it turns out
that we were creating it in our WebView (also our PageClient) before initializing
m_isVisible. Valgrind's memcheck tool was kind enough to catch this in 2 different contexts:

Conditional jump or move depends on uninitialised value(s)
   at 0x6F537F9: WebKit::WebPageProxy::viewStateDidChange(unsigned int) (WebPageProxy.cpp:1001)
   by 0x708B418: WebKit::WebView::setVisible(bool) (WebView.cpp:159)
   by 0x7089F93: NIXViewSetVisible (NIXView.cpp:112)
   by 0x40B1CF: MiniBrowser::MiniBrowser(_GMainLoop*, Options const&) (main.cpp:258)
   by 0x40ECE6: main (main.cpp:1115)

Conditional jump or move depends on uninitialised value(s)
   at 0x6F60C56: WebKit::WebPageProxy::updateBackingStoreDiscardableState() (WebPageProxy.cpp:4337)
   by 0x6F53984: WebKit::WebPageProxy::viewStateDidChange(unsigned int) (WebPageProxy.cpp:1048)
   by 0x708B3CA: WebKit::WebView::setFocused(bool) (WebView.cpp:148)
   by 0x7089F3E: NIXViewSetFocused (NIXView.cpp:102)
   by 0x40B1B7: MiniBrowser::MiniBrowser(_GMainLoop*, Options const&) (main.cpp:257)
   by 0x40ECE6: main (main.cpp:1115)

Reviewed by: Hugo Parente Lima