win: Fix build of headless/ with is_multi_dll=false.

headless_shell_child_lib transitively includes something from webrtc
(via content/browser) and it gets the include dir for that transitively via a
dep on blink if is_multi_dll is true. So things only work by accident there.

As a workaround, since we only depend on headless_shell_child_lib if
is_multi_dll is true, make the whole target conditionaly.

This remains questionable deps-wise (since we pull in the webrtc header
via content/browser headers but get the include flag for it via
the blink dep), but since multi dll hopefully goes away soon it'll fix itself.

Bug: 726150
Change-Id: I23908e640a8ba61ace00ca5189f80d88c1537f5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1866849
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706934}

headless/BUILD.gn

@@ -749,6 +749,7 @@ if (is_win) {
     }
   }
 
+  if (is_multi_dll_chrome) {
     # Headless library with child specific dependencies (e.g., renderer). This
     # is used when no browser depencendies are needed (e.g. chrome:child_dll).
     static_library("headless_shell_child_lib") {
@@ -766,7 +767,6 @@ if (is_win) {
         "//ui/base",
       ]
 
-    if (is_multi_dll_chrome) {
       defines = [ "CHROME_MULTIPLE_DLL_CHILD" ]
       sources += [
         "lib/browser/headless_web_contents_impl.cc",