Bug 1590050 - Preserve markup view selection in iframes after reload r=rcaliman,gl

Depends on D49940

To support this feature we perform two main changes
- the node actor exposes a getAllSelectors method, and the inspector now stores all selectors rather than just one
- the node actor exposes a waitForFrameLoad method, and the walkerFront findNodeFront helper uses it to make sure frames are loaded before querying a selector

Also added a test

Differential Revision: https://phabricator.services.mozilla.com/D49941

devtools/shared/inspector/css-logic.js

@@ -24,6 +24,11 @@ loader.lazyImporter(
   "findCssSelector",
   "resource://gre/modules/css-selector.js"
 );
+loader.lazyImporter(
+  this,
+  "findAllCssSelectors",
+  "resource://gre/modules/css-selector.js"
+);
 loader.lazyImporter(
   this,
   "getCssPath",
@@ -497,6 +502,16 @@ exports.prettifyCSS = prettifyCSS;
  */
 exports.findCssSelector = findCssSelector;
 
+/**
+ * Retrieve the array of CSS selectors corresponding to the provided node.
+ *
+ * The selectors are ordered starting with the root document and ending with the deepest
+ * nested frame. Additional items are used if the node is inside a frame or a shadow root,
+ * each representing the CSS selector for finding the frame or root element in its parent
+ * document.
+ */
+exports.findAllCssSelectors = findAllCssSelectors;
+
 /**
  * Get the full CSS path for a given element.
  * @returns a string that can be used as a CSS selector for the element. It might not