Copy debug PNaCl plugin in Dev builds of the chromoting webapp.

With this change the Debug version of the PNaCl plugin will be copied to
the webapp directory allowing to debug PNaCl plugin (using nacl-gdb).

Also updated build-webapp.py to process all *.jinja2 input files as jinja2
templates.

This patch was previously reverted because of a problem that should be
fixed now by https://codereview.chromium.org/872673006/ .

BUG=451643

Review URL: https://codereview.chromium.org/872633006

Cr-Commit-Position: refs/heads/master@{#314873}
4 files changed