Add failure messages to gin and js mojom encoder

Add failure messages to gin and js mojom encoder

I hit two failures while trying to use network_service
today, both of which would have been much easier to
catch with these two validators.

This CL adds validation of encoding array pointers.
We already validate in a few other places in codec.js
and clearly need many more.

This CL also makes it more clear in gin when type
coercion fails what argument (0-indexed) failed
to convert and what js type was seen in the process.

I started down the path of logging the c++ destination
type as well, but found I couldn't with our current
RTTI-less build.

R=abarth@chromium.org, hansmuller@chromium.org
BUG=

Committed: https://chromium.googlesource.com/external/mojo/+/4df7c4d89873e0efd93001d7fc6986e2aac0d732

[abarth-chromium, 2014-12-13 05:42:58]

abarth@chromium.org changed reviewers:
+ abarth@chromium.org

[abarth-chromium, 2014-12-13 05:42:58]

LGTM, but you'll need to land the gin changes in the Chromium repo.  Gin is
developed canonically there and changes we have locally in Mojo will just get
blown away in the next merge.

[hansmuller, 2014-12-15 16:56:17]

https://codereview.chromium.org/798163002/diff/1/mojo/public/js/codec.js
File mojo/public/js/codec.js (right):

https://codereview.chromium.org/798163002/diff/1/mojo/public/js/codec.js#newc...
mojo/public/js/codec.js:357: throw new Error(kErrorArray);
I suppose another indication that val isn't an array (if val.length was defined)
would be that val.length < 0. But shouldn't you be using |val instanceof Array|
here?

[eseidel, 2014-12-15 20:25:18]

I wasn't sure what the right pattern is.  I'm happy to move to instanceof array
if you'd rather.

[hansmuller, 2014-12-15 20:26:21]

On 2014/12/15 20:25:18, eseidel wrote:
> I wasn't sure what the right pattern is.  I'm happy to move to instanceof
array
> if you'd rather.

I think that's the right thing to do if you want to verify that something is an
array.

[eseidel, 2014-12-15 20:27:24]

What about array-like objects?  NodeList for example?

[eseidel, 2014-12-15 20:46:34]

Committed patchset #1 (id:1) manually as
4df7c4d89873e0efd93001d7fc6986e2aac0d732 (presubmit successful).

[hansmuller, 2014-12-15 21:54:09]

On 2014/12/15 20:27:24, eseidel wrote:
> What about array-like objects?  NodeList for example?

I'd rather require callers to convert array-like objects to real arrays than to
define a valid array as something for which .length is defined. Do we really
want to fix one obscure failure mode but leave callMojoArrayMethod({length:3})?

[eseidel, 2014-12-15 22:12:34]

Will post a patch to fix shortly.

[eseidel, 2014-12-15 23:53:34]

Posted https://codereview.chromium.org/803173003/