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Previously, given a device_id "plug:foo" alsa_util.cc would try to open
the control named ":foo". It will now open "foo".

This is safe because at this point we know we have already found a ':'
at pos1.

BUG=internal 35197382
TEST=manual
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2883313004
Cr-Commit-Position: refs/heads/master@{#472218}