Reland "Add about:flag for feature PdfIsolation""

This reverts commit abc55490658e0ec0bf2be019449b7248b923ce90.
This cl had to be reverted to allow the revert of its predecessor.
Now that the predecessor is relanded, it can be re-applied.

Original CL at https://chromium-review.googlesource.com/c/chromium/src/+/956488

TBR=jam@chromium.org

Bug: 809614
Change-Id: I7a4eceed9d8a7df5e7a60ae79b226eab8a0dd9ad
Reviewed-on: https://chromium-review.googlesource.com/961105
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542879}
4 files changed