PAGE_TARGETS_INVALID tells CFG (Control Flow Guard) to mark all
addresses as invalid indirect branch targets. This makes exploits more
difficult. The benefit is minor because most of the code in the Chrome
process doesn't use the CFG checks, but this will close off a few
weaknesses and is the direction we will want to go in eventually
anyway (with specific targets or call sites opted-in to allowing
calls, using SetProcessValidCallTargets).

PAGE_TARGETS_INVALID may ultimately cause CFG to not allocate memory -
that is implied by Windows Internals 7th Edition - and if that is
implemented then this change will save some modest amount of memory.

PAGE_TARGETS_INVALID was introduced in Windows 10 - according to
Windows Internals Part 1 7th Edition - prior to that it will cause
VirtualAlloc to fail.

Bug: chromium:870054
Change-Id: Ib1784fba37cc0ecb5fe5df595f1519531b3b3a20
Reviewed-on: https://chromium-review.googlesource.com/1186025
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55365}