Print Preview: Add some null checks for customMargins.

If the printing sticky settings somehow ended up with
"customMargins:null", then Print Preview code that assumes customMargins
values are always valid will fail while reading properties of null.

Change-Id: I8fb297e58050bf78605931c8aafb390035f486e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2102083
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750303}

chrome/browser/resources/print_preview/ui/margin_control_container.js

@@ -171,7 +171,7 @@ Polymer({
   /** @private */
   onMarginSettingsChange_() {
     const margins = this.getSettingValue('customMargins');
-    if (margins.marginTop === undefined) {
+    if (!margins || margins.marginTop === undefined) {
       // This may be called when print preview model initially sets the
       // settings. It sets custom margins empty by default.
       return;
@@ -204,8 +204,8 @@ Polymer({
     if (this.state === State.READY && this.resetMargins_ === null) {
       // Don't reset margins if there are sticky values. Otherwise, set them
       // to the document margins when the user selects custom margins.
-      this.resetMargins_ =
-          this.getSettingValue('customMargins').marginTop === undefined;
+      const margins = this.getSettingValue('customMargins');
+      this.resetMargins_ = !margins || margins.marginTop === undefined;
     }
   },