Add a `inject css` rule

Change-Id: I5d3051590bf9c0d205bd6cf1266bc8d68b1a3c95

src/content_script.ts

diff --git a/src/content_script.ts b/src/content_script.ts
index 3d5478f..a375e53 100644
--- a/src/content_script.ts
+++ b/src/content_script.ts
@@ -130,6 +130,13 @@
       const el = document.createElement('div');
       el.innerHTML = rule.destination;
       document.body.appendChild(el);
+    } else if (rule.operator === Operator.INJECT_CSS) {
+      onGerritReady().then(() => {
+        const link = document.createElement('link');
+        link.setAttribute('rel', 'stylesheet');
+        link.setAttribute('href', rule.destination);
+        document.head.appendChild(link);
+      });
     } else if (rule.operator === Operator.INJECT_JS_PLUGIN) {
       onGerritReady().then(() => {
         const link = document.createElement('script');

src/popup.ts

diff --git a/src/popup.ts b/src/popup.ts
index a61307d..2d737d0 100644
--- a/src/popup.ts
+++ b/src/popup.ts
@@ -313,6 +313,7 @@
   operators = [
     Operator.BLOCK,
     Operator.REDIRECT,
+    Operator.INJECT_CSS,
     Operator.INJECT_HTML_CODE,
     Operator.INJECT_JS_PLUGIN,
     Operator.INJECT_JS_MODULE_PLUGIN,

src/utils.ts

diff --git a/src/utils.ts b/src/utils.ts
index 50a6ffd..50d739f 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -20,6 +20,7 @@
 
 export function isInjectRule(rule: Rule) {
   return [
+    Operator.INJECT_CSS,
     Operator.INJECT_JS_MODULE_PLUGIN,
     Operator.INJECT_JS_PLUGIN,
     Operator.INJECT_HTML_CODE,
@@ -131,6 +132,7 @@
 export enum Operator {
   BLOCK = 'block',
   REDIRECT = 'redirect',
+  INJECT_CSS = 'injectCss',
   INJECT_HTML_CODE = 'injectHtmlCode',
   INJECT_JS_PLUGIN = 'injectJSPlugin',
   INJECT_JS_MODULE_PLUGIN = 'injectJSModule',