FileManagerUITest fix for disabled WebComponentsV0

The combination of loading html-imports.min.js but also having the
renderer parse and load imports for file:// URLs, causes polymer to
fail to load in FileManagerUITests.

Adding a check for toast.shadowRoot and creating one if required fixes
the test (crrev.com/c/2373967/15).

Bug: 1111393
Change-Id: I79decc2a58192598b57f7c485073edaea3a87e78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2386681Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803351}

ui/file_manager/file_manager/test/crostini_share.js

@@ -267,6 +267,9 @@ shareBase.testSharePaths = async (
 const createMockFilesAppToast = () => {
   const toast = document.querySelector('#toast');
 
+  if (!toast.shadowRoot) {
+    toast.attachShadow({mode: 'open'});
+  }
   toast.shadowRoot.innerHTML = `
     <div class="container" id="container" hidden>
       <div class="text" id="text" hidden></div>