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

@@ -749,24 +749,24 @@ if (is_win) {
     }
   }
 
-  # 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") {
-    sources = [
-      "app/headless_shell.cc",
-      "app/headless_shell.h",
-      "app/headless_shell_win.cc",
-      "public/headless_shell.h",
-    ]
-    deps = [
-      ":headless_renderer",
-      "//build:branding_buildflags",
-      "//content/public/child:child",
-      "//net",
-      "//ui/base",
-    ]
+  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") {
+      sources = [
+        "app/headless_shell.cc",
+        "app/headless_shell.h",
+        "app/headless_shell_win.cc",
+        "public/headless_shell.h",
+      ]
+      deps = [
+        ":headless_renderer",
+        "//build:branding_buildflags",
+        "//content/public/child:child",
+        "//net",
+        "//ui/base",
+      ]
 
-    if (is_multi_dll_chrome) {
       defines = [ "CHROME_MULTIPLE_DLL_CHILD" ]
       sources += [
         "lib/browser/headless_web_contents_impl.cc",