mw.ViewPageTarget: Clean up transform/restore logic

Merge methods:
* transformPage: transformSkinTabs + hideSiteNotice
* restorePage: restoreSkinTabs + restoreSiteNotice

Callers now consistently do both (e.g. #tearDownSurface only
called #restoreSkinTabs). This is also in preparation for
bug 43844.

Removed call from #tearDownSurface as that method is only
called from #deactivate which already calls #restorePage.

Clean up support check:
* Rename supportsStrictMode to supportsES5. Though the feature
  test asserts strict mode, we use it to check support for
  ES5. VisualEditor doesn't particularily care about strict
  mode itself.
* Reverse support if-statemement to return early instead of
  wrapping everything that follows. This makes it easier to
  see that we intent to abort if the current environment is
  known to be problematic. Also easier for code later on
  by not accidentally falling outside this block
* Follows-up aaa5ad254be12aa69.

Change-Id: Ia4b949d9c066a3f7b07217aa3d51de9908734e85
Merge "mw.ViewPageTarget: Clean up transform/restore logic"