Enable thin archives for ChromeOS builds

LLVM has been updated in the ChromeOS sysroot, so it should be safe to enable
thin archives now.

BUG=801925

Change-Id: I7dc2492937a26adc4f9af61abccbdb086e3a38cb
Reviewed-on: https://chromium-review.googlesource.com/1050980Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557336}

build/config/compiler/BUILD.gn

@@ -1588,11 +1588,7 @@ config("thin_archive") {
   # have a "thin archive" mode (it does accept -T, but it means truncating
   # archive names to 16 characters, which is not what we want).
   if ((is_posix && !is_nacl && !is_mac && !is_ios) || is_fuchsia) {
-    # TODO(thomasanderson): Enable on ChromeOS builds once
-    # https://crbug.com/829956 is fixed.
-    if (default_toolchain != "//build/toolchain/cros:target") {
-      arflags = [ "-T" ]
-    }
+    arflags = [ "-T" ]
   }
 }
 

build/toolchain/gcc_toolchain.gni

@@ -335,11 +335,7 @@ template("gcc_toolchain") {
       ar_wrapper =
           rebase_path("//build/toolchain/gcc_ar_wrapper.py", root_build_dir)
 
-      # TODO(thomasanderson): Remove the special ChromeOS case once
-      # https://crbug.com/829956 is fixed.
-      if (default_toolchain == "//build/toolchain/cros:target") {
-        extra_arflags = "rcsD"
-      } else if (current_os == "aix") {
+      if (current_os == "aix") {
         # We use slightly different arflags for AIX.
         extra_arflags = "-r -c -s"
       } else {