Do not perform a non-incremental compile when previous compile failed.

Do not perform a non-incremental compile when previous compile failed.

This reverts a not-fully-thought-out change to md5_check.py from
http://crrev.com/d8cef607861

Compiles fail for legitimate reasons, and we shouldn't delete md5
information in this case.

The concern behind that logic in the first case was that buggy --incremental
logic might be the reason for compile to fail. So, this commit also adds
a new environment variable to force all targets to be rebuilt:

FORCE_REBUILD=1

BUG=533442
Committed: https://crrev.com/a1ae0267dc973f87eecc864485be03f91613b86d
Cr-Commit-Position: refs/heads/master@{#351345}

[agrieve, 2015-09-29 15:01:02]

agrieve@chromium.org changed reviewers:
+ yfriedman@chromium.org

[agrieve, 2015-09-29 15:01:17]

On 2015/09/29 15:01:02, agrieve wrote:
> mailto:agrieve@chromium.org changed reviewers:
> + mailto:yfriedman@chromium.org

♤

[agrieve, 2015-09-29 15:02:06]

The CQ bit was checked by agrieve@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-29 15:02:31]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1374263002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1374263002/1

[commit-bot: I haz the power, 2015-09-29 16:45:45]

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

[commit-bot: I haz the power, 2015-09-29 16:45:46]

Dry run: This issue passed the CQ dry run.

[Yaron, 2015-09-29 16:48:48]

lgtm

[agrieve, 2015-09-29 17:19:35]

The CQ bit was checked by agrieve@chromium.org

[commit-bot: I haz the power, 2015-09-29 17:20:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1374263002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1374263002/1

[commit-bot: I haz the power, 2015-09-29 17:27:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-29 17:27:42]

Patchset 1 (id:??) landed as
https://crrev.com/a1ae0267dc973f87eecc864485be03f91613b86d
Cr-Commit-Position: refs/heads/master@{#351345}