Neutralize dangerous subresource files during Save Page.

Downloading a complete page using "Save as..." can result in downloading
hundreds of subresources. The user often isn't interested in accessing
individual resources directly while they are on disk. In addition,
scanning hundreds of files during a single save page operation isn't
currently practical.

In order to mitigate the potential risk of leaving dangerous files
around on the users' filesystem, this CL renames known dangerous files
with an additional ".download" extension. I.e. A subresource named
foo.exe would be saved as

The code review includes lists of file types that are known to be
affected by this change. Notable file types include .js, .swf, and
.class. As a side-effect of the rename, they will not receive the
correct MIME type when loaded via a file:// URL. The saved page should
still function correctly even with the renamed resources.,

Cr-Commit-Position: refs/heads/master@{#401729}
11 files changed