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The Trusty sysroot was only used for building Chromium for ARM.  Since
this is now the responsibility of the Jessie sysroot, the Trusty
sysroot can be removed.

R=thestig@chromium.org,sbc@chromium.org

Review-Url: https://codereview.chromium.org/2859323002
Cr-Commit-Position: refs/heads/master@{#469565}