[Downloads] Prevent executable files from being opened automatically.

[Downloads] Prevent executable files from being opened automatically.

Users can accidentally mark executable or other dangerous file types for
opening automatically upon download. This change prevents such file
types from being considered for auto open.

If the user already has any dangerous file types in the auto-open list,
those entries will be ignored on new browser sessions. Any action that
causes the persisted auto-open list to be updated will, as a
side-effect, premanently remove any dangerous file types that were
there.

Currently the list of file types that are excluded from auto open is
maintained by chrome/browser/download/download_extension.cc.

BUG=461858
TEST=(1) Download an .exe (on Windows) or .swf file.
     (2) Verify that the "Always open files of this type" option is
         disabled.

Review URL: https://codereview.chromium.org/1165893004

TBR=asanka@chromium.org

Cr-Commit-Position: refs/heads/master@{#334677}
(cherry picked from commit 39841e54180e2583dffa16fbbb9b99fd293821d0)

Committed: https://chromium.googlesource.com/chromium/src/+/ea65713eae7ad5977b6af590eb45e9551ea1fb2e

[asanka, 2015-07-10 15:53:11]

Committed patchset #1 (id:1) manually as
ea65713eae7ad5977b6af590eb45e9551ea1fb2e (presubmit successful).