Use a JS dictionary to initialize a the fields in a Mojo struct

Enable using a JS dictionary to initialize a the fields in a Mojo struct. For example, to initialize a URLRequest:

  var urlRequest = new loader.URLRequest({
    url: "http://www.cnn.com",
    method: "GET",
    auto_follow_redirects: true
  });

The value of unspecified fields is the Mojo default for the field's type. Values for fields that were not specified in the Mojom struct definition are ignored.

BUG=416163
Committed: https://crrev.com/25584136ce30ad399720cf1af27c4579810beeaf
Cr-Commit-Position: refs/heads/master@{#296410}

[hansmuller, 2014-09-23 23:06:10]

hansmuller@chromium.org changed reviewers:
+ mpcomplete@google.com

[hansmuller, 2014-09-23 23:26:33]

This is a small change, PTAL.

[Matt Perry, 2014-09-23 23:35:44]

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

[Matt Perry, 2014-09-23 23:35:44]

LGTM

[hansmuller, 2014-09-23 23:37:28]

The CQ bit was checked by hansmuller@chromium.org

[commit-bot: I haz the power, 2014-09-23 23:38:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/595063002/20001

[commit-bot: I haz the power, 2014-09-24 00:04:44]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-24 00:04:45]

Try jobs failed on following builders:
  ios_dbg_simulator on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)
  ios_rel_device on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device/bu...)
  ios_rel_device_ninja on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)

[hansmuller, 2014-09-24 00:21:15]

The CQ bit was checked by hansmuller@chromium.org

[commit-bot: I haz the power, 2014-09-24 00:22:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/595063002/20001

[commit-bot: I haz the power, 2014-09-24 00:27:49]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-24 00:27:50]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-09-24 00:27:51]

Try jobs failed on following builders:
  ios_dbg_simulator on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)
  ios_rel_device on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device/bu...)
  ios_rel_device_ninja on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)

[hansmuller, 2014-09-24 14:31:56]

The CQ bit was checked by hansmuller@chromium.org

[commit-bot: I haz the power, 2014-09-24 14:32:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/595063002/20001

[commit-bot: I haz the power, 2014-09-24 14:43:48]

Committed patchset #2 (id:20001) as 9dcdc9f334530b8910b1e4983227c9c13da1ece3

[commit-bot: I haz the power, 2014-09-24 14:44:28]

Patchset 2 (id:??) landed as
https://crrev.com/25584136ce30ad399720cf1af27c4579810beeaf
Cr-Commit-Position: refs/heads/master@{#296410}