Add support for a 'default' target to GN.

Add support for a 'default' target to GN.

Many build systems (including make and ninja) have the
concept of a "default" target that should be built if
you don't specify something explicitly to build: in
make, it's the first target found in the Makefile; in ninja
it is the target declared with the "default" directive, and
if that isn't found, the first target in the ninja file.

This CL adds the ability to set the "default" target
in the GN file; if it isn't set, we use GN's prior behavior,
which is to use "all" as the default target, and build
everything.

R=brettw@chromium.org
BUG=440644
Committed: https://crrev.com/576ecade9157d40a85f5b7b8b724199dedd2a006
Cr-Commit-Position: refs/heads/master@{#307770}

[Dirk Pranke, 2014-12-10 03:17:41]

Brett, I think this is as we've discussed.

I didn't add a test for this :(.

There doesn't appear to be any tests for nina_build_writer.cc already, and I
suspect that's because the way the NinjaBuildWriter is declared, it isn't easy
to override it and trap the output streams so that we can test things (since it
uses private constructors).

I can take a look at changing that, but I wasn't sure if you would want me to.

The change is straightforward enough that maybe it's okay that we don't have
tests, but I'm not super happy about it.

[brettw, 2014-12-10 18:02:26]

lgtm

[Dirk Pranke, 2014-12-10 18:06:22]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2014-12-10 18:07:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/784413004/1

[commit-bot: I haz the power, 2014-12-10 18:17:19]

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

[commit-bot: I haz the power, 2014-12-10 18:17:19]

Try jobs failed on following builders:
  linux_chromium_compile_dbg_32_ng on tryserver.chromium.linux
(http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[Dirk Pranke, 2014-12-10 20:48:58]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2014-12-10 20:49:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/784413004/1

[commit-bot: I haz the power, 2014-12-10 21:38:01]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-10 21:38:49]

Patchset 1 (id:??) landed as
https://crrev.com/576ecade9157d40a85f5b7b8b724199dedd2a006
Cr-Commit-Position: refs/heads/master@{#307770}

[Nico, 2014-12-10 23:14:32]

Re CL description:

"""
 in ninja
it is the target declared with the "default" directive, and
if that isn't found, the first target in the ninja file.
"""

This is not true. If there's no explicit "default", ninja builds all targets
without out edges by default: that is, all targets that are "final products".
That's equivalent to executing all edges in the build graph.

[Dirk Pranke, 2014-12-10 23:16:16]

On 2014/12/10 23:14:32, Nico wrote:
> Re CL description:
> 
> """
>  in ninja
> it is the target declared with the "default" directive, and
> if that isn't found, the first target in the ninja file.
> """
> 
> This is not true. If there's no explicit "default", ninja builds all targets
> without out edges by default: that is, all targets that are "final products".
> That's equivalent to executing all edges in the build graph.

Oh, I guess I misremembered. Thanks for the correction. Too bad the patch
already landed :(.