GN: Don't compile battor_agent, imagediff, bitmaptools on Android

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 76c24b996e883c784bd135a23bf073e441b08825..76bd1cb14b0938fc60810e67544dbdbb0939b56e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -196,9 +196,6 @@ group("both_gn_and_gyp") {
       "//third_party/mojo/src/mojo/edk/test:mojo_public_system_unittests",
       "//third_party/mojo/src/mojo/edk/test:mojo_public_utility_unittests",
       "//third_party/smhasher:pmurhash",
-      "//tools/battor_agent",
-      "//tools/imagediff($host_toolchain)",
-      "//tools/telemetry:bitmaptools($host_toolchain)",
       "//ui/display:display_unittests",
       "//ui/events:events_unittests",
       "//ui/gl:gl_unittests",
@@ -224,6 +221,15 @@ group("both_gn_and_gyp") {
     }
   }
 
+  # Tools that are relevant only on the host.
+  if (is_mac || is_win || is_linux) {
+    deps += [
+      "//tools/battor_agent",
+      "//tools/imagediff($host_toolchain)",
+      "//tools/telemetry:bitmaptools($host_toolchain)",

[Dirk Pranke, 2015/12/23 21:19:32]

I don't think this change is right. 

You definitely still need to build imagediff and bitmaptools when
target_os==android, you just need to build the host versions (as it says).

I suspect that's true for battor_agent as well, so may the right thing to do is
to change that to have ($host_toolchain)?

[agrieve, 2015/12/28 19:46:07]

Hmm, guess my brain wasn't working correctly :P. I've put back imagediff and
bitmaptools. I tried marking battor_agent as $host_toolchain, but that actually
fails to gn gen due to inconsistencies in how use_gconf is set. Rather than
fixing that, I took it as a sign that battor_agent isn't needed for is_android
and left it in this separate block.

[charliea (OOO until 10-5), 2016/01/04 15:38:37]

Yep - you're right in that battor agent is only needed for desktop builds. The
agent is a tracing tool that communicates from a desktop to a connected device
that's measuring power consumption.

+    ]
+  }
+
   deps += root_extra_deps
 
   if (enable_extensions) {