Linux: Add build flag to allow skipping the distro version check

This build flag is intended to be set by the "Build From Tarball" bot which does
an unbundled build.  This is necessary because some of the libraries that we
statically link (eg. harfbuzz) use features that are very new and not available
on all (or any) supported distros.

BUG=800977
R=thestig

Change-Id: Ie7d635effe148aae07a75b60221442b0c71f2a38
Reviewed-on: https://chromium-review.googlesource.com/933725Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538842}

chrome/installer/linux/BUILD.gn

@@ -16,6 +16,12 @@ if (current_cpu == "x86" || current_cpu == "x64") {
   import("//media/cdm/library_cdm/cdm_paths.gni")
 }
 
+declare_args() {
+  # Enables checks that ensure the packages are installable on all supported
+  # distributions.
+  enable_distro_version_check = true
+}
+
 assert(is_linux)
 
 packaging_files_executables = [
@@ -85,7 +91,7 @@ action_foreach("calculate_deb_dependencies") {
     target_cpu,
     "deb_{{source_name_part}}.deps",
   ]
-  if (is_desktop_linux && target_cpu == "x64") {
+  if (is_desktop_linux && target_cpu == "x64" && enable_distro_version_check) {
     args += [ "--distro-check" ]
   }
 }
@@ -129,7 +135,7 @@ action_foreach("calculate_rpm_dependencies") {
     "rpm_{{source_name_part}}.deps",
   ]
   args += rebase_path(packaging_files_shlibs, root_build_dir)
-  if (is_desktop_linux && target_cpu == "x64") {
+  if (is_desktop_linux && target_cpu == "x64" && enable_distro_version_check) {
     args += [ "--distro-check" ]
   }
 }