GN: add target_arch build arg for android

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # =============================================================================
 # BUILD FLAGS
 # =============================================================================
 #
 # This block lists input arguments to the build, along with their default
 # values. GN requires listing them explicitly so it can validate input and have
@@ skipping 53 matching lines @@
 
   # Compile for Thread Sanitizer to find threading bugs.
   is_tsan = false
 
   if (os == "chromeos") {
     # Allows the target toolchain to be injected as arguments. This is needed
     # to support the CrOS build system which supports per-build-configuration
     # toolchains.
     cros_use_custom_toolchain = false
   }
+
+  if (os == "android") {
+    # Architecture of the target device.

[brettw, 2014/10/16 19:19:07]

Can you add here something to the effect of "This will be equal to the cpu_arch
of the default toolchain. When checking the CPU architecture for source files
and build dependencies, you should almost always use cpu_arch instead. cpu_arch
is the architecture of the current toolchain, and using it allows cross-compiles
(compiling the same target for multiple toolchains in the same build) to be
supported.

[cjhopman, 2014/10/16 21:29:59]

Done.

+    target_arch = "arm"
+  }
 }
 
 # =============================================================================
 # OS DEFINITIONS
 # =============================================================================
 #
 # We set these various is_FOO booleans for convenience in writing OS-based
 # conditions.
 #
 # - is_android, is_chromeos, is_ios, and is_win should be obvious.
@@ skipping 83 matching lines @@
 if (is_win) {
   # Always use 32-bit on Windows, even when compiling on a 64-bit host OS,
   # unless the override flag is specified.
   if (force_win64) {
     cpu_arch = "x64"
   } else {
     cpu_arch = "x86"
   }
 }
 
+if (is_android) {
+  # TODO(cjhopman): enable this assert once bots are updated to not set
+  # cpu_arch.
+  #assert(cpu_arch == build_cpu_arch, "Android device target architecture should
+  #    be set with 'target_arch', not 'cpu_arch'")
+  cpu_arch = target_arch
+}
+
 # =============================================================================
 # SOURCES FILTERS
 # =============================================================================
 #
 # These patterns filter out platform-specific files when assigning to the
 # sources variable. The magic variable |sources_assignment_filter| is applied
 # to each assignment or appending to the sources variable and matches are
 # automatcally removed.
 #
 # Note that the patterns are NOT regular expressions. Only "*" and "\b" (path
@@ skipping 496 matching lines @@
       if (defined(invoker.output_extension)) { output_extension = invoker.output_extension }
       if (defined(invoker.output_name)) { output_name = invoker.output_name }
       if (defined(invoker.public)) { public = invoker.public }
       if (defined(invoker.public_configs)) { public_configs = invoker.public_configs }
       if (defined(invoker.public_deps)) { public_deps = invoker.public_deps }
       if (defined(invoker.sources)) { sources = invoker.sources }
       if (defined(invoker.visibility)) { visibility = invoker.visibility }
     }
   }
 }