`testRunner.dumpAsText(true)` doesn't dump pixels

[Vitaly Dyackhov]

`LayoutTests/animations/animation-offscreen-to-onscreen.html` for example uses this snippet:
```
if (window.testRunner) {
    window.testRunner.dumpAsText(true);
}
```

But `WebKitTestRunner` only dumps text but not pixels:
```
WebKitBuild/Release/bin/WebKitTestRunner LayoutTests/animations/animation-offscreen-to-onscreen.html
Gdk-Message: 15:30:20.976: Window 0x5605b1a30790 is a temporary window without parent, application will not be able to position it on screen.
Content-Type: text/plain
PASS - "webkitTransform" property for "box" element at 0.5s saw something close to: 1,0,0,1,100,0

#EOF
#EOF
#EOF
```

[Vitaly Dyackhov]

Pull request: https://github.com/WebKit/WebKit/pull/15105

[Alexey Proskuryakov]

I could swear we had a bug about this, but I cannot find it.

[Radar WebKit Bug Importer]

<rdar://problem/111388446>