Make MB clobber build directories when switching between GYP and GN.

Make MB clobber build directories when switching between GYP and GN.

GYP and GN put generated files in different places inside the build
directory. This means that if you do a GYP build and then a GN build
into the same directory, you may get unpredictable results depending
on which generated files are picked up at what times.

In order to avoid this, we modify MB to keep track of whether a
given build directory has build files generated by GYP or GN; if there
is a mismatch (like when we flip a bot from GYP to GN), we clobber the
build directory and start from scratch to be safe.

In addition, the first time we enable MB on a bot, we will not know
what the existing build directory contains; to be safe, we do a clobber
in this situation as well.

R=brettw@chromium.org
BUG=

Committed: https://crrev.com/c161aa945c811b260f7f2981e592f63d16c3a106
Cr-Commit-Position: refs/heads/master@{#348705}

[Dirk Pranke, 2015-09-12 00:49:41]

https://codereview.chromium.org/1338123002/diff/1/tools/mb/mb.py
File tools/mb/mb.py (right):

https://codereview.chromium.org/1338123002/diff/1/tools/mb/mb.py#newcode296
tools/mb/mb.py:296: 'gyp_defines': '',
this was a bug caught by the new unit tests.

[brettw, 2015-09-12 02:08:25]

https://codereview.chromium.org/1338123002/diff/1/tools/mb/mb.py
File tools/mb/mb.py (right):

https://codereview.chromium.org/1338123002/diff/1/tools/mb/mb.py#newcode856
tools/mb/mb.py:856: # In other places in chromium, we often have to retry this
command
But doesn't shutil still work? I don't see the need for special-casing this.

[Dirk Pranke, 2015-09-12 15:39:49]

On 2015/09/12 02:08:25, brettw wrote:
> https://codereview.chromium.org/1338123002/diff/1/tools/mb/mb.py
> File tools/mb/mb.py (right):
> 
> https://codereview.chromium.org/1338123002/diff/1/tools/mb/mb.py#newcode856
> tools/mb/mb.py:856: # In other places in chromium, we often have to retry this
> command
> But doesn't shutil still work? I don't see the need for special-casing this.

Past experience has not been good w/ shutil.rmtree() on windows. It might work,
but
I'm not sure I want to take that chance and then have to debug it later.

[brettw, 2015-09-12 22:34:49]

okay, whatever. LGTM

[brettw, 2015-09-12 22:34:49]

okay, whatever. LGTM

[brettw, 2015-09-12 22:34:50]

okay, whatever. LGTM

[Dirk Pranke, 2015-09-14 18:29:27]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2015-09-14 18:30:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1338123002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1338123002/1

[commit-bot: I haz the power, 2015-09-14 20:21:19]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-14 20:22:06]

Patchset 1 (id:??) landed as
https://crrev.com/c161aa945c811b260f7f2981e592f63d16c3a106
Cr-Commit-Position: refs/heads/master@{#348705}

[commit-bot: I haz the power, 2015-09-23 12:36:41]

Patchset 1 (id:??) landed as
https://crrev.com/c161aa945c811b260f7f2981e592f63d16c3a106
Cr-Commit-Position: refs/heads/master@{#348705}