Add alternative method to use a version/git hash from a file.

- This is necessary for Chromium builds from tarballs where git is no longer available. This gives Chromium another option to create the version file when creating the tarball to accomplish the same thing.

Bug: dawn:549
Change-Id: Iffb4bf694b0df1306dd92939353422e5115346a7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/94043
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

scripts/dawn_overrides_with_defaults.gni

@@ -85,3 +85,7 @@ if (!defined(dawn_vulkan_validation_layers_dir)) {
 if (!defined(dawn_abseil_dir)) {
   dawn_abseil_dir = "//third_party/abseil-cpp"
 }
+
+if (!defined(dawn_version_file)) {
+  dawn_version_file = ""
+}