Docked Web Inspectors are flickery

[kfix]

upstream changes: WebKit/WebKit@551a9e2

• https://github.com/WebKit/webkit/blob/aefe74816c3a4db1b389893469e2d9475bbc0fd9/Source/WebKit2/UIProcess/mac/WebInspectorProxyMac.mm#L809
• https://github.com/WebKit/webkit/blob/aefe74816c3a4db1b389893469e2d9475bbc0fd9/Source/WebKit2/UIProcess/mac/WebInspectorProxyMac.mm#L725

log of NSLayout when inspector docked to bottom of floating browser window

<build/macosx-x86_64-apple-macosx10.11/apps/MacPin.app/Contents/MacOS/MacPin> modules/MacPin/AppScriptRuntime.swift[REPL()]:> 2015-12-31 15:40:40.971 MacPin[22268:266644] Clicked on overlapping visualized constraints: (
    "<NSAutoresizingMaskLayoutConstraint:0x7f8ebc02b2a0 h=-&- v=--& V:[WKWebInspectorWKWebView:0x7f8ebc120420(300)]> (Actual Distance - pixels):600",
    "<NSAutoresizingMaskLayoutConstraint:0x7f8ebc14d8e0 h=--& v=--& V:[WKView:0x7f8eba562020(784)]> (Actual Distance - pixels):1568",
    "<NSAutoresizingMaskLayoutConstraint:0x7f8ebc09a5f0 h=-&- v=--& H:|-(0)-[WKWebInspectorWKWebView:0x7f8ebc120420]   (Names: '|':MacPin.MPWebView:0x7f8eba540d50'kfix/MacPin \U00b7 GitHub' )> (Actual Distance - pixels):0",
    "<NSAutoresizingMaskLayoutConstraint:0x7f8ebc14dae0 h=--& v=--& H:|-(0)-[WKView:0x7f8eba562020]   (Names: '|':MacPin.MPWebView:0x7f8eba540d50'kfix/MacPin \U00b7 GitHub' )> (Actual Distance - pixels):0"
)