<https://webkit.org/b/119836> [GTK] ChromeClient::paint is susceptible to system time changes

Reviewed by Martin Robinson.

Instead of using WTF::currentTime() as the value of the last display occurrence,
WTF::monotonicallyIncreasingTime() should be used. The latter is not affected by
the changes is the system's time, which can cause a halt in the display updating.

* WebCoreSupport/ChromeClientGtk.cpp:
(WebKit::ChromeClient::paint):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154380 268f45cc-cd09-0410-ab3c-d52691b4dbfc