Handle chrome.fileSystem.chooseEntry() errors in the PDF Viewer.

When the user cancels a save operation, chrome.fileSystem.chooseEntry()
does not pass back a valid entry. Use chrome.runtime.lastError to check
for this condition.

Change-Id: I54e655ec74b4d3c07f06cf981677751a2d38acab
Reviewed-on: https://chromium-review.googlesource.com/c/1446867Reviewed-by: Dan Beam <dbeam@chromium.org>
Reviewed-by: dstockwell <dstockwell@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627693}

chrome/browser/resources/pdf/pdf_viewer.js

@@ -1148,6 +1148,14 @@ PDFViewer.prototype = {
 
     chrome.fileSystem.chooseEntry(
         {type: 'saveFile', suggestedName: fileName}, entry => {
+          if (chrome.runtime.lastError) {
+            if (chrome.runtime.lastError.message != 'User cancelled') {
+              console.log(
+                  'chrome.fileSystem.chooseEntry failed: ' +
+                  chrome.runtime.lastError.message);
+            }
+            return;
+          }
           entry.createWriter(writer => {
             writer.write(
                 new Blob([result.dataToSave], {type: 'application/pdf'}));