Adds a pref and a policy to decide if PDFs should always be opened externally.

If set this pref disables the PDF viewer and instead download the file and
automatically opens the external PDF viewer.

UI for controlling the pref will be added in subsequent CL.

BUG=628014
TEST=unit_tests
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2369353002
Cr-Commit-Position: refs/heads/master@{#423189}
13 files changed