Add a file type description when saving edited PDFs.

When calling chrome.fileSystem.chooseEntry() to save edited PDFs, there
is an option for the file types to accept. For each file type, the
description is optional. The API documentation [1] says "if not present,
a description will be automatically generated", but the generated
description ended up being an empty string in this case. Fix this by
manually setting the file type description.

[1] https://developer.chrome.com/apps/fileSystem

Bug: 1114127
Change-Id: If3440fcd13a152bb55e8f80193b4960103776cd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343782Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796427}

chrome/browser/resources/pdf/pdf_viewer.js

@@ -944,7 +944,7 @@ export class PDFViewerElement extends PDFViewerBaseElement {
     chrome.fileSystem.chooseEntry(
         {
           type: 'saveFile',
-          accepts: [{extensions: ['pdf']}],
+          accepts: [{description: '*.pdf', extensions: ['pdf']}],
           suggestedName: fileName
         },
         entry => {