Do not pass use_prebuilt_mojo_shell to gn.

Do not pass use_prebuilt_mojo_shell to gn.

Should fix "Chromium Mojo Linux"'s "gn gen" error:
http://build.chromium.org/p/chromium.mojo/builders/Chromium%20Mojo%20Linux/builds/8374/steps/gn/logs/stdio

ERROR at the command-line "--args":1:25: Build argument has no effect.
use_prebuilt_mojo_shell=true is_debug=false cpu_arch="x64" use_goma=true goma_dir="/mnt/data/b/build/goma"
                        ^---
The variable "use_prebuilt_mojo_shell" was set as a build argument
but never appeared in a declare_args() block in any buildfile.

Removing the command-line override of the gn arg should be okay, now that
a Mojo roll that added the self-contained arg logic to
mojo/public/mojo.gni <https://codereview.chromium.org/917433003>;.

BUG=NONE
TEST=No gn gen errors.
R=ppi@chromium.org,dpranke@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=294406

[msw, 2015-03-11 18:16:13]

Hey Dirk and Przemyslaw, please take a look; thanks!
(I get a *lot* of seemingly unrelated presubmit errors...)

[Dirk Pranke, 2015-03-11 18:52:45]

This change looks fine, if you actually want us to not use the prebuilt mojo
shell. It looks like https://codereview.chromium.org/917433003 actually sets it
to be false by default. Is that right?

[msw, 2015-03-11 19:04:58]

On 2015/03/11 18:52:45, Dirk Pranke wrote:
> This change looks fine, if you actually want us to not use the prebuilt mojo
> shell. It looks like https://codereview.chromium.org/917433003 actually sets
it
> to be false by default. Is that right?

No, we still want to use the prebuilt mojo shell, but I think this should be
correct. The current build/module_args/mojo.gni no longer specifies
|use_prebuilt_mojo_shell| nor |build_mojo_shell_from_source|, after
<https://codereview.chromium.org/921763002>. See
<https://chromium.googlesource.com/chromium/src/+/master/build/module_args/moj...>
and
<https://chromium.googlesource.com/chromium/src/+/master/third_party/mojo/src/...>.
So the logic in the third_party gni file should set |use_prebuilt_mojo_shell| to
true (on linux, and |build_mojo_shell_from_source| is not defined).

[Dirk Pranke, 2015-03-11 19:10:42]

Ah, okay, I hadn't seen that other change.

lgtm.

[msw, 2015-03-11 19:11:12]

The CQ bit was checked by msw@chromium.org

[commit-bot: I haz the power, 2015-03-11 19:11:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1003453002/20001

[commit-bot: I haz the power, 2015-03-11 19:15:34]

Committed patchset #2 (id:20001) as
http://src.chromium.org/viewvc/chrome?view=rev&revision=294406