Use jinja2 templating engine for Python code generators

Use jinja2 templating engine for Python code generators

Instead of manually building C++ source files, we can use a templating engine
like jinja2. This should make it easier to add new code generators and edit
existing ones.

Comparing the output from the templates here and the previous generators, the
only differences are minor whitespace changes (http://pastebin.com/XPicZmPk),
for the sake of making the templates more readable.

BUG=

[eseidel, 2013-05-06 00:39:04]

I don't speak jinja very well.  But this seems reasonable.  I'll chat with
abarth tomorrow when he's back at work.  I expect he'll have thoughts since
we've both been looking at this perl -> python conversion for our
autogeneration.

[abarth-chromium, 2013-05-06 18:27:32]

Nice!  This looks much better than hand-rolling our templates.  My main comment
is about how to integrate jinja into the build.

Does the Chromium project use any Python templating systems already?  If so, we
might want to use the same one...

https://codereview.chromium.org/14905002/diff/1/Source/core/scripts/make_even...
File Source/core/scripts/make_event_factory.py (right):

https://codereview.chromium.org/14905002/diff/1/Source/core/scripts/make_even...
Source/core/scripts/make_event_factory.py:70: return
template_expander.applytemplate("../dom/EventFactory.cpp.tmpl", params)
Should we receive the path to EventFactory.cpp.tmpl on the command line from
GYP?

https://codereview.chromium.org/14905002/diff/1/Source/core/scripts/template_...
File Source/core/scripts/template_expander.py (right):

https://codereview.chromium.org/14905002/diff/1/Source/core/scripts/template_...
Source/core/scripts/template_expander.py:33: autoinstallerpath =
"../../../Tools/Scripts/webkitpy/common/system"
Woah, we shouldn't have a dependency across the world like this.

https://codereview.chromium.org/14905002/diff/1/Source/core/scripts/template_...
Source/core/scripts/template_expander.py:37:
installer.install(url="https://pypi.python.org/packages/source/J/Jinja2/Jinja2-2.6.tar.gz#md5=1c49a8825c993bfdcf55bb36897d28a2",
url_subpath="Jinja2-2.6/jinja2")
Can we just check jinja into third_party?  We don't want to autoinstall stuff
during compile.  That's what DEPS is for.

[abarth-chromium, 2013-05-06 18:28:54]

+haraken as an FYI.  We might consider using something like this in the bindings
code generator if/when we switch to Python.

[eseidel, 2013-05-08 01:22:54]

other than abarth's complaints this looks totally reasonable.  Thank you for
working on this!

[eseidel, 2013-05-10 08:09:15]

Looks very reasonable.  Assuming this generates the same output as the current
scripts, lgtm.

[Timothy Loh, 2013-05-10 08:17:19]

On 2013/05/10 08:09:15, Eric Seidel wrote:
> Looks very reasonable.  Assuming this generates the same output as the current
> scripts, lgtm.

I think you wanted to send that message to
https://codereview.chromium.org/15095002/. I'll close this one since I'm
converting the generators separately now and will need multiple CLs.

[haraken, 2013-05-10 08:44:50]

We should use this for CodeGeneratorV8.py in the future :)