Print Preview: Update header/footer availability based on doc margins

Updating the custom margins setting will always result in a
corresponding update to the document margins reported from the backend,
so adjust header/footer availability based on these margins rather than
the custom margins setting value. This prevents unnecessary extra calls
to the observer, and fixes a bug where the custom margins setting being
initially empty caused the header/footer checkbox to disappear.

Bug: 1051810
Change-Id: I851947d3e6b102154910c401ead79aed1666f43e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2063869Reviewed-by: Daniel Hosseinian <dhoss@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742744}

chrome/browser/resources/print_preview/data/model.js

@@ -491,8 +491,7 @@ Polymer({
         'documentSettings.isPdf, documentSettings.hasCssMediaStyles, ' +
         'documentSettings.hasSelection)',
     'updateHeaderFooterAvailable_(' +
-        'margins, settings.margins.value, ' +
-        'settings.customMargins.value, settings.mediaSize.value)',
+        'margins, settings.margins.value, settings.mediaSize.value)',
   ],
 
   /** @private {boolean} */
@@ -798,28 +797,19 @@ Polymer({
     }
 
     // Otherwise, availability depends on the margins.
-    let available = false;
     const marginsType =
         /** @type {!MarginsType} */ (this.getSettingValue('margins'));
-    switch (marginsType) {
-      case MarginsType.DEFAULT:
-        available = !this.margins ||
+    if (marginsType === MarginsType.NO_MARGINS) {
+      return false;
+    }
+
+    if (marginsType === MarginsType.MINIMUM) {
+      return true;
+    }
+
+    return !this.margins ||
         this.margins.get(CustomMarginsOrientation.TOP) > 0 ||
         this.margins.get(CustomMarginsOrientation.BOTTOM) > 0;
-        break;
-      case MarginsType.NO_MARGINS:
-        break;
-      case MarginsType.MINIMUM:
-        available = true;
-        break;
-      case MarginsType.CUSTOM:
-        const margins = this.getSettingValue('customMargins');
-        available = margins.marginTop > 0 || margins.marginBottom > 0;
-        break;
-      default:
-        break;
-    }
-    return available;
   },
 
   /**

chrome/test/data/webui/print_preview/model_settings_availability_test.js

@@ -421,33 +421,46 @@ suite('ModelSettingsAvailabilityTest', function() {
     // Default margins + letter paper + HTML page.
     assertTrue(model.settings.headerFooter.available);
 
+    // Custom margins initializes with customMargins undefined and margins
+    // values matching the defaults.
+    model.set('settings.margins.value', MarginsType.CUSTOM);
+    assertTrue(model.settings.headerFooter.available);
+
     // Set margins to NONE
     model.set('settings.margins.value', MarginsType.NO_MARGINS);
     assertFalse(model.settings.headerFooter.available);
 
+    // Set margins to MINIMUM
+    model.set('settings.margins.value', MarginsType.MINIMUM);
+    assertTrue(model.settings.headerFooter.available);
+
     // Custom margins of 0.
     model.set('settings.margins.value', MarginsType.CUSTOM);
     model.set(
         'settings.customMargins.value',
         {marginTop: 0, marginLeft: 0, marginRight: 0, marginBottom: 0});
+    model.set('margins', new Margins(0, 0, 0, 0));
     assertFalse(model.settings.headerFooter.available);
 
     // Custom margins of 36 -> header/footer available
     model.set(
         'settings.customMargins.value',
         {marginTop: 36, marginLeft: 36, marginRight: 36, marginBottom: 36});
+    model.set('margins', new Margins(36, 36, 36, 36));
     assertTrue(model.settings.headerFooter.available);
 
     // Zero top and bottom -> header/footer unavailable
     model.set(
         'settings.customMargins.value',
         {marginTop: 0, marginLeft: 36, marginRight: 36, marginBottom: 0});
+    model.set('margins', new Margins(0, 36, 0, 36));
     assertFalse(model.settings.headerFooter.available);
 
     // Zero top and nonzero bottom -> header/footer available
     model.set(
         'settings.customMargins.value',
         {marginTop: 0, marginLeft: 36, marginRight: 36, marginBottom: 36});
+    model.set('margins', new Margins(0, 36, 36, 36));
     assertTrue(model.settings.headerFooter.available);
 
     // Small paper sizes