[Windows MSVC CFG]  Link with CFG only on executables.

Adjust GN configs to only link with Control Flow Guard for executables.
This will still support CFG compiled into Microsoft system DLLs, in all
Chromium processes (e.g. chrome.exe).

This should prevent sporadic official clang build problems in DLLs, related
to CFG.  Further investigation/fixes to follow.

TEST=CFGSupportTests.MsIndirectFailure in sbox_integration_tests suite.
BUG=584575,708098

Review-Url: https://codereview.chromium.org/2813823006
Cr-Commit-Position: refs/heads/master@{#464475}
2 files changed