fix(form-field): label gap not being calculated when switching to outline dynamically

Fixes the form field gap not being calculated if the field switches to the `outline` appearance after init.

Fixes #11653.

Author: crisbeto

src/lib/form-field/form-field.ts

@@ -148,6 +148,11 @@ export class MatFormField extends _MatFormFieldMixinBase
     return this._appearance || this._defaultOptions && this._defaultOptions.appearance || 'legacy';
   }
   set appearance(value: MatFormFieldAppearance) {
+    // If we're switching to `outline` from another appearance, we have to recalculate the gap.
+    if (value !== this._appearance && value === 'outline') {
+      this._initialGapCalculated = false;
+    }
+
     this._appearance = value;
   }
   _appearance: MatFormFieldAppearance;