[Android] WebUI: Exclude unnecessary build tasks from Android and iOS

Chromium change:
https://github.com/chromium/chromium/commit/c996ee52684e17a1e4685e7d7ac3eaa377be76a8

[Reland] WebUI: Exclude unnecessary build tasks from Android and iOS …
…builds.

Added assertions to make sure that cr_elements/BUILD.gn and
cr_components/BUILD.gn are not loaded at all in Android/iOS builds.
These folders hold Polymer elements, which are not relevant for
Android/iOS since Polymer is not used (and not shipped) on those
platforms.

Those assertions revealed a bunch of places were unnecessary work was
triggered on Android/iOS builds, and had to be fixed.

These were originally discovered as part of migrating various shared
WebUI elements to TypeScript (through bot failures).

Bug: 1189595

browser/ui/sidebar/BUILD.gn

@@ -53,10 +53,13 @@ source_set("sidebar") {
     "//chrome/browser/cart:mojo_bindings",
     "//chrome/browser/new_tab_page/modules/drive:mojo_bindings",
     "//chrome/browser/new_tab_page/modules/task_module:mojo_bindings",
-    "//chrome/browser/ui/webui/new_tab_page:mojo_bindings",
     "//chrome/browser/ui/webui/realbox:mojo_bindings",
     "//ui/webui/resources/js/browser_command:mojo_bindings",
   ]
+
+  if (!is_android && !is_ios) {
+    deps += [ "//chrome/browser/ui/webui/new_tab_page:mojo_bindings" ]
+  }
 }
 
 source_set("browser_tests") {