By inserting "print(current_toolchain)" to sysroot.gni, I found that arm sysroot is needed by "//native_client/src/trusted/service_runtime/linux/toolchain:nacl_bootstrap_arm".

I know nothing about nacl, but it seems nacl_bootstrap_arm toolchain is defined in src/native_client/src/trusted/service_runtime/linux/toolchain/BUILD.gn.

Probably you should just install the arm sysroot and everything should just work fine?
At least, in my case, it worked and I can build chrome which can run on an arm device.

Another possible fix is changing sysroot.gni to use target_sysroot even when current_toolchain != default_toolchain if current_cpu == target_cpu.

Always using target_sysroot if current_cpu == target_cpu may not work as it can result in building host binaries with target_sysroot if host_cpu==target_cpu.

Yet another possible fix is fixing nacl toolchain to use target_sysroot.

Made changes to use target_sysroot when current_cpu == target_cpu && current_os == target_os: https://codereview.chromium.org/1901353002/

This also needs a nacl side change: https://codereview.chromium.org/1902683004/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3a1ce21a7cc8c9301173b595764b6545785804e8

commit 3a1ce21a7cc8c9301173b595764b6545785804e8
Author: hashimoto <hashimoto@chromium.org>
Date: Thu Apr 21 03:03:03 2016

Use target_sysroot when building for target OS on target CPU

nacl's bootstrap toolchain should also use target_sysroot.

Nacl side change: https://codereview.chromium.org/1902683004/

BUG= 604971 
TEST=Remove build/linux/debian_wheezy_arm-sysroot, enter chrome-sdk with an arm board, gn gen --args=$GN_ARGS out/gn && ninja -C out/gn

Review URL: https://codereview.chromium.org/1901353002

Cr-Commit-Position: refs/heads/master@{#388668}

[modify] https://crrev.com/3a1ce21a7cc8c9301173b595764b6545785804e8/build/config/sysroot.gni

Both changes landed.
Waiting for nacl-deps-roller to roll native_client.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9ce787a6dd2aeafb0801145a66d31babdd3a0297

commit 9ce787a6dd2aeafb0801145a66d31babdd3a0297
Author: nacl-deps-roller <nacl-deps-roller@chromium.org>
Date: Fri Apr 22 16:02:38 2016

Roll src/native_client/ f640889e8..da301fb59 (8 commits).

https://chromium.googlesource.com/native_client/src/native_client.git/+log/f640889e8f9f..da301fb59c80

$ git log f640889e8..da301fb59 --date=short --no-merges --format='%ad %ae %s'
2016-04-22 stichnot Update revision for PNaCl
2016-04-21 stichnot PNaCl: Update subzero revision in pnacl/COMPONENT_REVISIONS
2016-04-20 anmittal Alphabetize and clean top-level gn BUILD files
2016-04-20 hashimoto Set bootstrap toolchain's toolchain_os to target_os
2016-04-20 smklein PNaCl Dynamic Linking: Loader Exporting / Importing TLS Variables.
2016-04-20 smklein PNaCl: Update llvm revision in pnacl/COMPONENT_REVISIONS
2016-04-19 phosek PNaCl: Update binutils revision in pnacl/COMPONENT_REVISIONS
2016-04-19 phosek Allow multiple symbol definitions in dynamic linking tests

BUG= 604971 

CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_dbg_32_ng,linux_nacl_sdk_build
TBR=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/1911393002

Cr-Commit-Position: refs/heads/master@{#389125}

[modify] https://crrev.com/9ce787a6dd2aeafb0801145a66d31babdd3a0297/DEPS

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