Example: embedded webextension overlay (#249)

* Simple Legacy Overlay Addon with an Embedded WebExtension.

* fix: fixed eslint errors on embedded-webextension-overlay example

embedded-webextension-overlay/chrome.manifest

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+content my-overlay-addon content/
+overlay chrome://browser/content/browser.xul chrome://my-overlay-addon/content/overlay.xul

embedded-webextension-overlay/content/init.js

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+/* globals Components, dump */
+
+{
+  const addonId = "my-overlay-addon@me";
+  const {
+    AddonManager,
+  } = Components.utils.import("resource://gre/modules/AddonManager.jsm", {});
+
+  AddonManager.getAddonByID(addonId, addon => {
+    const baseURI = addon.getResourceURI("/");
+
+    const {
+      LegacyExtensionsUtils,
+    } = Components.utils.import("resource://gre/modules/LegacyExtensionsUtils.jsm");
+
+    const myOverlayEmbeddedWebExtension = LegacyExtensionsUtils.getEmbeddedExtensionFor({
+      id: addonId, resourceURI: baseURI,
+    });
+
+    myOverlayEmbeddedWebExtension.startup().then(({browser}) => {
+      dump(`${addonId} - embedded webext started\n`);
+      browser.runtime.onMessage.addListener(msg => {
+        dump(`${addonId} - received message from embedded webext ${msg}\n`);
+      });
+    }).catch(err => {
+      Components.utils.reportError(
+        `${addonId} - embedded webext startup failed: ${err.message} ${err.stack}\n`
+      );
+    });
+  });
+}

embedded-webextension-overlay/content/overlay.xul

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+<?xml version="1.0"?>
+<overlay id="myOverlayAddon" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <script src="chrome://my-overlay-addon/content/init.js"></script>
+</overlay>

embedded-webextension-overlay/install.rdf

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+<?xml version="1.0"?>
+<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+    xmlns:em="http://www.mozilla.org/2004/em-rdf#">
+
+  <Description about="urn:mozilla:install-manifest">
+    <em:id>my-overlay-addon@me</em:id>
+    <em:version>1.0.1</em:version>
+    <em:name>My Legacy Overlay Addon</em:name>
+
+    <em:type>2</em:type>
+    <em:multiprocessCompatible>true</em:multiprocessCompatible>
+
+    <em:targetApplication>
+      <Description>
+        <!-- firefox -->
+        <em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
+        <em:minVersion>51.0</em:minVersion>
+        <em:maxVersion>*</em:maxVersion>
+      </Description>
+    </em:targetApplication>
+
+  </Description>
+</RDF>

embedded-webextension-overlay/webextension/background.js

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+console.log("Embedded WebExtension", window.location.href);
+
+browser.runtime.sendMessage("embedded_webext -> overlay addon container");

embedded-webextension-overlay/webextension/manifest.json

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+{
+  "manifest_version": 2,
+  "name": "Overlay Addon WebExtension",
+  "version": "1.0.1",
+  "description": "test embedding a webextension in a legacy overlay addon",
+
+  "background": {
+    "scripts": ["background.js"]
+  }
+}