Fix GN os, cpu defaults for Android builds.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 782226158745fe5ad90e46efab4aad2cc2b02348..a554f12dc5b427ddbff8a0fff1116eb9bc6cbbdd 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -37,6 +37,12 @@ if (target_cpu == "") {
     # If cpu_arch is defined, it was set in an args file and needs to be
     # used for backwards-compatibility.
     target_cpu = cpu_arch
+  } else if (target_os == "android") {
+    # If we're building for Android, we should assume that we want to
+    # build for ARM by default, not the host_cpu (which is likely x64).
+    # This allows us to not have to specify both target_os and target_cpu
+    # on the command line.
+    target_cpu = "arm"
   } else {
     target_cpu = host_cpu
   }
@@ -108,17 +114,6 @@ declare_args() {
     cros_use_custom_toolchain = false
   }
 
-  # TODO(dpranke): Replace target_arch with target_cpu.
-
-  # Architecture of the target device. For Android builds, this will be
-  # equal to the current_cpu of the default toolchain. When checking
-  # the CPU architecture for source files and build dependencies you
-  # should almost alway use current_cpu instead. current_cpu is the
-  # architecture of the current toolchain and allows cross-compiles
-  # (compiling the same target for multiple toolchains in the same build)
-  # to work.
-  target_arch = "arm"

[Dirk Pranke, 2015/02/20 20:06:50]

I think it's perhaps best to just cut straight to getting users to specify
target_cpu instead of target_arch, if they need to do an x86 android build (or
some other variant). I'm not even sure if any non-arm Android build actually
works at the moment, so I don't know if anyone is actually using non-arm values,


It would not be hard to support target_arch for backwards compatibility for some
time, though, if that would be preferred.

[cjhopman, 2015/02/20 20:28:19]

Make them use target_cpu sgtm.

-
   # TODO(brettw) remove this flag (and therefore enable linking all targets) on
   # Windows when we have sufficient bot capacity. In the meantime, you can
   # enable linking for local compiles.
@@ -216,18 +211,6 @@ if (current_os == "win") {
 }
 
 # =============================================================================
-# CPU ARCHITECTURE
-# =============================================================================
-
-if (is_android) {
-  # TODO(cjhopman): enable this assert once bots are updated to not set
-  # current_cpu.
-  #assert(current_cpu == host_cpu, "Android device target architecture should
-  #    be set with 'target_arch', not 'current_cpu'")
-  current_cpu = target_arch

[Dirk Pranke, 2015/02/20 20:06:50]

This was the thing I broke in https://codereview.chromium.org/913373002/ ; this
should've been target_cpu, not current_cpu.

-}
-
-# =============================================================================
 # SOURCES FILTERS
 # =============================================================================
 #