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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}