Teach install-build-deps.sh --arm how to install the arm root image.

build/install-build-deps.sh

Index: build/install-build-deps.sh
diff --git a/build/install-build-deps.sh b/build/install-build-deps.sh
index fb3833e67a1e8d4359cd2d791516167b0bf17ece..0d3718ccc172b87792e733ad0203a3657676db46 100755
--- a/build/install-build-deps.sh
+++ b/build/install-build-deps.sh
@@ -249,6 +249,42 @@ else
   exit 100
 fi
 
+# Install arm root image
+if test "$do_inst_arm" = "1"; then
+  # As well as the arm toolchain packages we also need a recent arm root
+  # image to build against (using --sysroot).  We could construct this
+  # from scratch based on the current state or precise/arm but for
+  # consistency we currently use a pre-built root image which was constructed
+  # for building trusted NaCl code.
+  CHROME_ROOT="$(dirname ${BASH_SOURCE[0]})"
+  CHROME_ROOT="$(dirname ${CHROME_ROOT})"
+  SYSROOT="${CHROME_ROOT}/arm-sysroot"
+  TC_URL_PREFIX=https://commondatastorage.googleapis.com/nativeclient-archive2/toolchain
+  TC_REV=8001
+  TC_URL=${TC_URL_PREFIX}/${TC_REV}/naclsdk_linux_arm-trusted.tgz
+
+  INSTALL_ROOT="yes"
+  STAMP="${SYSROOT}/.stamp"
+  if [ -f "${STAMP}" ]; then
+    if [ "${TC_URL}" = $(cat ${STAMP}) ]; then
+      INSTALL_ROOT="no"
+    fi
+  fi
+
+  if [ $INSTALL_ROOT = "no" ]; then
+    echo "ARM root image already up-to-date."
+  else
+    echo "Installing ARM root image."
+    mkdir -p ${SYSROOT}
+    tarball=${SYSROOT}/naclsdk_linux_arm-trusted.tgz
+    set -x
+    curl -L ${TC_URL} -o ${tarball}
+    tar xf ${tarball} -C ${SYSROOT}
+    rm ${tarball}
+    echo -n "${TC_URL}" > "${STAMP}"
+  fi
+fi
+
 # Install 32bit backwards compatibility support for 64bit systems
 if [ "$(uname -m)" = "x86_64" ]; then
   if test "$do_inst_lib32" != "1"