Add messages on assertions.

Add messages on assertions.

Without a message, you can't identify where the assertion was triggered,
except via the javascript console.

BUG=397019
Committed: https://crrev.com/479fb92ffb01afee1b56b8c7d96d9d775a1a1cbd
Cr-Commit-Position: refs/heads/master@{#297775}

[Anand Mistry (off Chromium), 2014-10-02 00:52:09]

amistry@chromium.org changed reviewers:
+ dbeam@chromium.org

[Dan Beam, 2014-10-02 01:18:19]

doesn't std{out,err} include the line number in the output?

[Anand Mistry (off Chromium), 2014-10-02 01:32:39]

On 2014/10/02 01:18:19, Dan Beam wrote:
> doesn't std{out,err} include the line number in the output?

Nope. It says:
[22970:22970:0929/153425:INFO:CONSOLE(28)] "Uncaught Error: Assertion failed",
source: chrome://resources/js/util.js (28)

Which:
1. Says nothing about where the assertion came from.
2. Is misleading, since the line is in assert.js, but looks like util.js since
it gets inlined using <include>

[Dan Beam, 2014-10-02 01:40:22]

oh yeah, fair enough.  lgtm

[Anand Mistry (off Chromium), 2014-10-02 01:49:28]

The CQ bit was checked by amistry@chromium.org

[commit-bot: I haz the power, 2014-10-02 01:50:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/620913003/1

[commit-bot: I haz the power, 2014-10-02 03:03:14]

Committed patchset #1 (id:1) as 3c14c7769541d5289c4dac6ad0afd9e485c3741e

[commit-bot: I haz the power, 2014-10-02 03:03:53]

Patchset 1 (id:??) landed as
https://crrev.com/479fb92ffb01afee1b56b8c7d96d9d775a1a1cbd
Cr-Commit-Position: refs/heads/master@{#297775}