set dawn_use_x11 = false for headless builds

headless_shell build with build/args/headless.gn should not
depend on x11, and https://crrev.com/c/2368534
brought x11 via dawn to all linux builds.

Bug: 976495
Change-Id: I65c0371f97e2c7eeda5381f890d52c8b06c60b34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2473444Reviewed-by: Peter Kvitek <kvitekp@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817311}

build/args/headless.gn

@@ -48,3 +48,4 @@ use_pangocairo = false
 
 # TODO(1096425): Remove this once use_x11 goes away.
 use_x11 = false
+dawn_use_x11 = false