Fixes a bug were UKM metrics were not being logged for autofill on iOS

autofill::AutofillManager passes FormData.main_frame_origin to the
autofill::AutofillMetrics::FormInteractionsUkmLogger. This property was not
being set in autofill_agent.mm, therefore no metrics were being logged.

Bug: 812070
Change-Id: I1e337ae6af575fc6182e5b360e44f12427d3d5be
Reviewed-on: https://chromium-review.googlesource.com/917534Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536678}

components/autofill/ios/browser/autofill_agent.mm

@@ -422,6 +422,9 @@ void GetFormAndField(autofill::FormData* form,
       return NO;
     }
 
+    // main_frame_origin is used for logging UKM.
+    form.main_frame_origin = url::Origin::Create(pageURL);
+
     // Action is optional.
     base::string16 action;
     formData->GetString("action", &action);