tools/gn: Fix errors found by Facebook's flint tool.

tools/gn: Fix errors found by Facebook's flint tool.

Example of warnings/errors/advices:
tools/gn/exec_process.h:5: Missing include guard. If you are ABSOLUTELY
sure that you don't want an include guard, then include a comment
containing override-include-guard in lieu of an include guard.
tools/gn/ninja_binary_target_writer.cc:50: Single-argument constructor
'IncludeWriter(PathOutput& path_output)' may inadvertently be used as a
type conversion constructor. Prefix the function with the 'explicit'
keyword to avoid this, or add an /* implicit */ comment to suppress this
warning.
tools/gn/standard_out.cc:98: Advice: Prefer `nullptr' to `NULL' in new
C++ code.  Unlike `NULL', `nullptr' can't accidentally be used in
arithmetic or as an integer. See
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2431.pdf for
details.
tools/gn/tutorial/hello.cc:7: The associated header file of .cpp files
should be included before any other includes.
tools/gn/target_generator.h:16: Warning: Base class TargetGenerator has
virtual functions but a public non-virtual destructor.

The errors were found with the following command line:

$ flint --recusive tools/gn

And cleaned up until silenced with:
$ gn gen out-gn
$ ninja -C out-gn gn gn_unittests

BUG=None
TEST=see above
R=scottmg@chromium.org,brettw@chromium.org

Committed: https://crrev.com/dc6279e30a3549ecb2fcab0596e05dc59d355d52
Cr-Commit-Position: refs/heads/master@{#318639}

[brettw, 2015-03-01 18:47:43]

brettw@chromium.org changed reviewers:
+ brettw@chromium.org

[brettw, 2015-03-01 18:47:44]

Super! LGTM. It's nice the Facebook tool matches Google-style so closely.

[tfarina, 2015-03-01 19:47:32]

The CQ bit was checked by tfarina@chromium.org

[tfarina, 2015-03-01 19:47:33]

The patchset sent to the CQ was uploaded after l-g-t-m from brettw@chromium.org
Link to the patchset: https://codereview.chromium.org/964203002/#ps20001 (title:
"fix chromium-style plugin errors")

[commit-bot: I haz the power, 2015-03-01 19:47:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/964203002/20001

[commit-bot: I haz the power, 2015-03-01 19:52:58]

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

[commit-bot: I haz the power, 2015-03-01 19:52:59]

Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[tfarina, 2015-03-01 22:06:28]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-03-01 22:07:33]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/964203002/20001

[commit-bot: I haz the power, 2015-03-01 22:12:40]

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

[commit-bot: I haz the power, 2015-03-01 22:12:41]

Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[tfarina, 2015-03-01 23:37:31]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-03-01 23:37:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/964203002/20001

[commit-bot: I haz the power, 2015-03-01 23:43:39]

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

[commit-bot: I haz the power, 2015-03-01 23:43:40]

Try jobs failed on following builders:
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)

[tfarina, 2015-03-02 02:05:27]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-03-02 02:05:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/964203002/20001

[commit-bot: I haz the power, 2015-03-02 03:07:23]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-03-02 03:08:13]

Patchset 2 (id:??) landed as
https://crrev.com/dc6279e30a3549ecb2fcab0596e05dc59d355d52
Cr-Commit-Position: refs/heads/master@{#318639}

[scottmg, 2015-03-02 03:11:28]

Cool! I'll have to try that tool.