Mojom: Blanket change to mark all pointer/handle fields as nullable.

Mojom: Blanket change to mark all pointer/handle fields as nullable.

The purpose is to make those APIs have the same semantics as before. And
then we can selectively turn fields into non-nullable.

Mojom files that are not included in this change:
- surface-related files: the owner (jamesr) has landed some non-nullable
  fixes. It is possible that he has fully reviewed them. (I will confirm
  with him.)
- test files: have been examined.

BUG=324170
TEST=None

Committed: https://crrev.com/7182448c92676e62ee5f19be1545de7a86eae012
Cr-Commit-Position: refs/heads/master@{#291862}

[yzshen1, 2014-08-25 04:11:06]

Hi, Darin.

I spent some miserable time inspecting the mojom files and corresponding code,
trying to selectively turn fields back into nullable. It didn't turn out very
well.

A better solution seems to be (1) preserve the old behavior for now (2) let the
authors of those files make the decision. (And sorry I probably should have done
it this way earlier.)

Therefore, if you think it makes sense, I would like to land this CL, and then
create some bugs asking authors to help review/change these files. 

Please let me know if you have better ideas. Thanks!

[darin (slow to review), 2014-08-25 18:17:12]

OK, LGTM

We'll surely have a round of API review before it is all set and done ;-)

You should mention this change on mojo-dev@

[yzshen1, 2014-08-25 18:22:53]

On 2014/08/25 18:17:12, darin wrote:
> OK, LGTM
> 
> We'll surely have a round of API review before it is all set and done ;-)
> 
> You should mention this change on mojo-dev@

Thank you, Darin.

I will do so.

[yzshen1, 2014-08-26 06:24:14]

The CQ bit was checked by yzshen@chromium.org

[commit-bot: I haz the power, 2014-08-26 06:26:11]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/yzshen@chromium.org/502853002/1

[commit-bot: I haz the power, 2014-08-26 07:09:51]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  mac_chromium_rel_swarming on tryserver.chromium.mac
(http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[commit-bot: I haz the power, 2014-08-26 07:45:38]

Committed patchset #1 (1) as c9237148b4c572e76e6eb7364be437bb00adfc8d

[commit-bot: I haz the power, 2014-09-10 02:41:14]

Patchset 1 (id:??) landed as
https://crrev.com/7182448c92676e62ee5f19be1545de7a86eae012
Cr-Commit-Position: refs/heads/master@{#291862}