Update StyleManager specs to address deprecated ::shadow selectors

As of Chromium 63, ::shadow CSS selectors have been fully removed:

https://developers.google.com/web/updates/2017/10/remove-shadow-piercing

As a result, any usage of ::shadow in a stylesheet seems to get removed as soon
as the stylesheet is applied to an HTMLStyleElement.  Since we have a couple of tests
that expect ::shadow selectors to be present, we must remove those expectations
since those selectors do not appear any longer.

spec/style-manager-spec.js

@@ -57,7 +57,6 @@ describe('StyleManager', () => {
           atom-text-editor::shadow .class-1, atom-text-editor::shadow .class-2 { color: red }
           atom-text-editor::shadow > .class-3 { color: yellow }
           atom-text-editor .class-4 { color: blue }
-          another-element::shadow .class-5 { color: white }
           atom-text-editor[data-grammar*=\"js\"]::shadow .class-6 { color: green; }
           atom-text-editor[mini].is-focused::shadow .class-7 { color: green; }
         `)
@@ -69,7 +68,6 @@ describe('StyleManager', () => {
           'atom-text-editor.editor .class-1, atom-text-editor.editor .class-2',
           'atom-text-editor.editor > .class-3',
           'atom-text-editor .class-4',
-          'another-element::shadow .class-5',
           'atom-text-editor[data-grammar*="js"].editor .class-6',
           'atom-text-editor[mini].is-focused.editor .class-7'
         ])
@@ -136,13 +134,6 @@ describe('StyleManager', () => {
         ).toEqual(['atom-text-editor .class-1, atom-text-editor .class-2'])
       })
 
-      it('does not transform CSS rules with invalid syntax', () => {
-        styleManager.addStyleSheet("atom-text-editor::shadow .class-1 { font-family: inval'id }")
-        expect(Array.from(styleManager.getStyleElements()[0].sheet.cssRules).map((r) => r.selectorText)).toEqual([
-          'atom-text-editor::shadow .class-1'
-        ])
-      })
-
       it('does not throw exceptions on rules with no selectors', () => {
         styleManager.addStyleSheet('@media screen {font-size: 10px}', {
           context: 'atom-text-editor'