[infra] Make gclient sync quiet when nothing is updated.

The 7zip dependency is only used on Windows. Use gclient's OS-specific
hook feature to only download it while on Windows. This avoids the
download from google cloud storage from needlessly saying it is doing
nothing.

There's no reason to point out the sysroot is already up to date as this
makes gclient output several lines of text with zero information.

Change-Id: Id1c212c8a951612c413e9834867fe90f6dca6a20
Reviewed-on: https://dart-review.googlesource.com/c/94752
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Jonas Termansen <sortie@google.com>

DEPS

@@ -436,21 +436,6 @@ hooks = [
       Var('dart_root') + "/third_party/firefox_jsshell",
     ],
   },
-  {
-    "name": "7zip",
-    "pattern": ".",
-    "action": [
-      "download_from_google_storage",
-      "--no_auth",
-      "--no_resume",
-      "--bucket",
-      "dart-dependencies",
-      "--platform=win32",
-      "--extract",
-      "-s",
-      Var('dart_root') + "/third_party/7zip.tar.gz.sha1",
-    ],
-  },
   {
     "name": "front_end_benchmark_data",
     "pattern": ".",
@@ -511,3 +496,23 @@ hooks = [
     'action': ['python', 'sdk/build/vs_toolchain.py', 'update'],
   },
 ]
+
+hooks_os = {
+  "win": [
+    {
+      "name": "7zip",
+      "pattern": ".",
+      "action": [
+        "download_from_google_storage",
+        "--no_auth",
+        "--no_resume",
+        "--bucket",
+        "dart-dependencies",
+        "--platform=win32",
+        "--extract",
+        "-s",
+        Var('dart_root') + "/third_party/7zip.tar.gz.sha1",
+      ],
+    },
+  ]
+}

build/linux/sysroot_scripts/install-sysroot.py

@@ -131,8 +131,6 @@ def InstallSysroot(target_platform, target_arch):
   if os.path.exists(stamp):
     with open(stamp) as s:
       if s.read() == url:
-        print '%s %s sysroot image already up to date: %s' % \
-            (target_platform, target_arch, sysroot)
         return
 
   print 'Installing Debian %s %s root image: %s' % \