https://bugs.webkit.org/show_bug.cgi?id=77875

Reviewed by Yury Semikhatsky.

Source/WebCore:

This change introduces InspectorHistory::Action that encapsulates all DOM modifications
initiated by the inspector. There is a way to undo these actions up until the undoable
state marker.

Tests: inspector/elements/undo-dom-edits-2.html
       inspector/elements/undo-dom-edits.html
       inspector/styles/undo-add-property.html
       inspector/styles/undo-change-property.html
       inspector/styles/undo-property-toggle.html

* CMakeLists.txt:
* GNUmakefile.list.am:
* Target.pri:
* WebCore.gypi:
* WebCore.vcproj/WebCore.vcproj:
* WebCore.xcodeproj/project.pbxproj:
* inspector/Inspector.json:
* inspector/InspectorAllInOne.cpp:
* inspector/InspectorCSSAgent.cpp:
(InspectorCSSAgent::StyleSheetAction):
(WebCore::InspectorCSSAgent::StyleSheetAction::StyleSheetAction):
(WebCore::InspectorCSSAgent::StyleSheetAction::perform):
(WebCore::InspectorCSSAgent::Style...