[Zucchini] Experimentally remove Zucchini capability from setup.exe.

This is an experiment to test incremental update framework, to prepare
for Zucchini launch in Stable, where the "old" setup.exe uses Courgette
to patch itself to the "new" setup.exe, which then uses Zucchini to
patch chrome.7z. In this CL, we remove Zucchini from setup.exe, so that
incremental install would revert setup.exe to the old state. Then after
a few releases (also an opportunity to gather metrics for Courgette),
reverting this CL would simulate Stable launch.

Bug: 729154
Change-Id: I4962b0f0df3dfbc32c39a663b559b84801b1af44
Reviewed-on: https://chromium-review.googlesource.com/c/1259471Reviewed-by: Samuel Huang <huangs@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596392}

chrome/installer/setup/buildflags.gni

@@ -5,5 +5,5 @@
 declare_args() {
   # Specify if the Zucchini patcher features should be included in setup.exe.
   # See //components/zucchini for more information.
-  use_zucchini = true
+  use_zucchini = false
 }