Define mojo_build_mojo_shell_from_source = true

Define mojo_build_mojo_shell_from_source = true

Chromium builds its own mojo_shell from forked source.

This logic was originally changed incorrectly:
  https://codereview.chromium.org/1049993002/diff/170001/third_party/mojo/src/mojo/public/mojo.gni
That was clobbered in the latest Mojo->Chromium roll:
  https://codereview.chromium.org/1082343002/diff/40001/third_party/mojo/src/mojo/public/mojo.gni

BUG=NONE
TEST=mojo_shell binary built from chromium repo.
R=jam@chromium.org
TBR=scottmg@chromium.org

Committed: https://crrev.com/f8d7df392c96b8b7bf5ed95cd31176cebddb3889
Cr-Commit-Position: refs/heads/master@{#325322}

[msw, 2015-04-15 21:34:52]

Hey John, please take a look; thanks!

[msw, 2015-04-15 21:38:04]

msw@chromium.org changed reviewers:
+ scottmg@chromium.org

[msw, 2015-04-15 21:40:32]

+scottmg for OWNERS; landing for green tree, unborked ToT.

[msw, 2015-04-15 21:40:41]

The CQ bit was checked by msw@chromium.org

[commit-bot: I haz the power, 2015-04-15 21:41:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1087333002/1

[commit-bot: I haz the power, 2015-04-15 22:06:28]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-15 22:08:19]

Patchset 1 (id:??) landed as
https://crrev.com/f8d7df392c96b8b7bf5ed95cd31176cebddb3889
Cr-Commit-Position: refs/heads/master@{#325322}

[jam, 2015-04-15 22:45:09]

lgtm