Enable gn check for all build targets.

This CL removes the `check_targets` entry from the .gn file which
will enable checking all targets. Two of the GN targets have had
missing dependencies added.

The `gn/abseil-cpp/BUILD.gn` was referenced against the Chromium
BUILD.gn for abseil to make sure the `public_deps` and `deps`
match in the two versions. The paths are re-written to match
the Dawn paths.

Bug: dawn:1372
Change-Id: I371e17ec9647e6627429be5c81eb7985da3cc5b3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86861
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn/samples/BUILD.gn

@@ -34,6 +34,7 @@ static_library("utils") {
   # Export all of these as public deps so that `gn check` allows includes
   public_deps = [
     "${dawn_root}/src/dawn:cpp",
+    "${dawn_root}/src/dawn:proc_shared",
     "${dawn_root}/src/dawn/common",
     "${dawn_root}/src/dawn/native",
     "${dawn_root}/src/dawn/utils",

src/dawn/tests/BUILD.gn

@@ -186,6 +186,7 @@ dawn_test("dawn_unittests") {
     "${dawn_root}/src/dawn/native:static",
     "${dawn_root}/src/dawn/utils",
     "${dawn_root}/src/dawn/wire",
+    "${dawn_root}/third_party/gn/abseil-cpp:strings",
   ]
 
   # Add internal dawn native config for internal unittests.