[Payments] Implement payment instrument icons

components/payments/mojom/payment_app.mojom

Index: components/payments/mojom/payment_app.mojom
diff --git a/components/payments/mojom/payment_app.mojom b/components/payments/mojom/payment_app.mojom
index f0bd4dd975e8eff45872a1b7ba57cb1cbdae10c8..e5d101dc7d12923298428b17c2e04391a7bd06cf 100644
--- a/components/payments/mojom/payment_app.mojom
+++ b/components/payments/mojom/payment_app.mojom
@@ -14,10 +14,18 @@ enum PaymentHandlerStatus {
   NOT_FOUND,
   NO_ACTIVE_WORKER,
   STORAGE_OPERATION_FAILED,
+  FETCH_INSTRUMENT_ICON_FAILED,
+};
+
+struct ImageObject {
+  string src;

[please use gerrit instead, 2017/06/08 21:34:10]

use mojom.url.GURL instead.

[gogerald1, 2017/06/09 14:23:24]

Why using mojom.url.GURL? PaymentInstruments has get method in the spec which
should return exact content used in set method.
https://w3c.github.io/payment-handler/#paymentinstruments-interface

So it looks better to store original representation of them directly, instead
convert them back and forth. 

[please use gerrit instead, 2017/06/09 14:52:53]

A strict IPC interface is better for security. (This data will always be a URL.)
In fact, using the GURL object up-and-down the full stack that passes around
this variable is preferable.

See "url.mojom.Url top_level_origin" below for example.

[gogerald1, 2017/06/09 18:50:55]

as talked offline

+  string sizes;

[please use gerrit instead, 2017/06/08 21:34:10]

This is never used. Discard, please. If you need it, then use an array of
integer pairs.

struct ImageSize {
  int32_t height;
  int32_t width;
}

array<ImageSize> sizes;

[gogerald1, 2017/06/09 14:23:24]

This is used by get interface. We could decode it when using it later. And the
usage is expected one time usage when fetching and decoding the instrument icon.

[please use gerrit instead, 2017/06/09 14:52:53]

All additional parameters increase the attack surface for IPC messages. Adding a
parameter that's not used increases the attack surface without any benefits.
Therefore, parameters should be added at the time when they are being used. 

[gogerald1, 2017/06/09 18:50:56]

Done.

+  string type;

[please use gerrit instead, 2017/06/08 21:34:10]

This is never used. Discard, please. If you need it, use an enum instead.

[gogerald1, 2017/06/09 14:23:23]

ditto

[gogerald1, 2017/06/09 18:50:55]

Done.

 };
 
 struct PaymentInstrument {
   string name;
+  array<ImageObject> icons;
   array<string> enabled_methods;
   string stringified_capabilities;
 };