https://bugs.webkit.org/show_bug.cgi?id=120715

Reviewed by Tim Horton.

In WebKitTestRunner, snapshots obtained via windowSnapshotImage() were
sometimes blank if a previous test triggered compositing mode, and the
current test or reference did not require compositing. This happened
because the UI process didn't wait for the web process to complete
its compositing mode switch before snapshotting. Fix by calling
WKPageForceRepaint() before we take the snapshot; this is async,
so we have to spin the runloop for a while.

Remove the Qt/EFL code that does the same thing.

* WebKitTestRunner/TestInvocation.cpp:
(WTR::TestInvocation::forceRepaintDoneCallback):
(WTR::TestInvocation::dumpResults):
* WebKitTestRunner/TestInvocation.h:
* WebKitTestRunner/cairo/TestInvocationCairo.cpp:
(WTR::TestInvocation::dumpPixelsAndCompareWithExpected):
* WebKitTestRunner/qt/TestInvocationQt.cpp:
(WTR::TestInvocation::dumpPixelsAndCompareWithExpected):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@155140 268f45cc-cd09-0410-ab3c-d52691b4dbfc