Add Linux build packaging targets to GN

Changes the version processing templates to not implicitly include the chrome version .rc file. This was something I've been meaning to do for a while which was forced by this patch, because it wants to call version.py with no sources or template file. I also had to remove some checks that either a template file or sources were defined (I wasn't aware this was valid to do when I wrote these checks).

The above-mentioned change necessitated referencing the version template file from a number of existing calls.

The version script now explicitly sets the output file with -o. Previously this was the implicit second argument, but this didn't work if there was no template file. -o is the same and it's better to be more explicit anyway.

Added copy rules for xdg-mime related scripts which are required by the installer.

Added a meta "installer" target which links to the Linux installers. This will also be a good place to hook up the Windows installer when we write it.

BUG=525839
TBR=thestig@chromium.org (linux installer), ddorwin@chromium.org (widevine)

Review URL: https://codereview.chromium.org/1311543003

Cr-Original-Commit-Position: refs/heads/master@{#346279}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a874dccb5c0b7ccfe803a798a585351ce5efccc7

config/chrome_build.gni

@@ -17,3 +17,11 @@ declare_args() {
   # on Windows.
   is_multi_dll_chrome = is_win && !is_component_build
 }
+
+# Refers to the subdirectory for branding in various places including
+# chrome/app/theme.
+if (is_chrome_branded) {
+  branding_path_component = "google_chrome"
+} else {
+  branding_path_component = "chromium"
+}