jinja2: make templates outside of // work again

jinja2: make templates outside of // work again

A recent CL -- http://crrev.com/1002313002 -- made the jinja wrapper script
barf on templates located outside CHROMIUM_SRC (the used Jinja2 loader
disallows template paths with '..' components). While not actively used
currently in the Chromium build, it is a somewhat useful feature of the
wrapper that used to work.

Fix by instatiating the jinja template loader with a configurable path as the
search directory, rather than a hardcoded CHROMIUM_SRC. Usages with templates
outside CHROMIUM_SRC now must pass an appropriate --loader-base-dir value
to the wrapper script.

R=cjhopman,jbudorick
BUG=

Committed: https://crrev.com/65b1b8d1d29b68df077dfaec40d3fd4e8961758e
Cr-Commit-Position: refs/heads/master@{#324597}

[newt (away), 2015-04-09 20:59:55]

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

[newt (away), 2015-04-09 20:59:55]

lgtm

[cjhopman, 2015-04-09 21:29:07]

lgtm

[tsniatowski, 2015-04-10 04:51:33]

The CQ bit was checked by tsniatowski@opera.com

[commit-bot: I haz the power, 2015-04-10 04:51:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1077553002/1

[commit-bot: I haz the power, 2015-04-10 06:45:27]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-10 06:46:19]

Patchset 1 (id:??) landed as
https://crrev.com/65b1b8d1d29b68df077dfaec40d3fd4e8961758e
Cr-Commit-Position: refs/heads/master@{#324597}

[tsniatowski, 2015-04-10 06:58:17]

Thanks :)