[Telemetry] Only add android_tools' adb to the path if it exists.
[Telemetry] Only add android_tools' adb to the path if it exists.

This directory is only checked out when target_os=['android']. So we can't
assume that adb will always be in the chrome checkout.

TBR=nduca@chromium.org
BUG=None
TEST=./tools/perf/run_multipage_benchmarks --browser=list

Review URL: https://codereview.chromium.org/11820035

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@175864 0039d316-1c4b-4281-b951-d872f2087c98

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