[GeckoView] Bundle the Firefox printing code in the viewer (bug 1810111)

This may not be enough, on its own, to completely fix [bug 1810111](https://bugzilla.mozilla.org/show_bug.cgi?id=1810111) however it's impossible for printing to work in GeckoView without this patch.

gulpfile.js

@@ -267,10 +267,9 @@ function createWebpackConfig(
       for (const key in viewerAlias) {
         viewerAlias[key] = "web/stubs-geckoview.js";
       }
-    } else {
-      viewerAlias["web-print_service"] = "web/firefox_print_service.js";
     }
     viewerAlias["web-com"] = "web/firefoxcom.js";
+    viewerAlias["web-print_service"] = "web/firefox_print_service.js";
   }
   const alias = { ...basicAlias, ...viewerAlias };
   for (const key in alias) {

web/viewer-geckoview.css

@@ -218,3 +218,58 @@ dialog :link {
 @page {
   margin: 0;
 }
+
+#printContainer {
+  display: none;
+}
+
+@media print {
+  body {
+    background: rgba(0, 0, 0, 0) none;
+  }
+  body[data-pdfjsprinting] #outerContainer {
+    display: none;
+  }
+  body[data-pdfjsprinting] #printContainer {
+    display: block;
+  }
+  #printContainer {
+    height: 100%;
+  }
+  /* wrapper around (scaled) print canvas elements */
+  #printContainer > .printedPage {
+    page-break-after: always;
+    page-break-inside: avoid;
+
+    /* The wrapper always cover the whole page. */
+    height: 100%;
+    width: 100%;
+
+    display: flex;
+    flex-direction: column;
+    justify-content: center;
+    align-items: center;
+  }
+
+  #printContainer > .xfaPrintedPage .xfaPage {
+    position: absolute;
+  }
+
+  #printContainer > .xfaPrintedPage {
+    page-break-after: always;
+    page-break-inside: avoid;
+    width: 100%;
+    height: 100%;
+    position: relative;
+  }
+
+  #printContainer > .printedPage canvas,
+  #printContainer > .printedPage img {
+    /* The intrinsic canvas / image size will make sure that we fit the page. */
+    max-width: 100%;
+    max-height: 100%;
+
+    direction: ltr;
+    display: block;
+  }
+}

web/viewer-geckoview.js

@@ -14,6 +14,7 @@
  */
 
 import "web-com";
+import "web-print_service";
 import { RenderingStates, ScrollMode, SpreadMode } from "./ui_utils.js";
 import { AppOptions } from "./app_options.js";
 import { LinkTarget } from "./pdf_link_service.js";