The calling code has been hand-modified to, wherever possible, create
vectors of StringPieces (rather than strings) for use with
base::JoinString. This reduces unnecessary string copying in the calling
code (sometimes updating surrounding logic to also use StringPieces).

BUG=348241
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/2695883003
Cr-Commit-Position: refs/heads/master@{#459983}