Fixes --output-format default case.
Fixes --output-format default case.

From python optparse docs for action='append':

"It also means that if the default value is non-empty, the default
elements will be present in the parsed value for the option, with any
values from the command line appended after those default values:"

So, to actually create a default, we have to set it later in
CreateResults. Unfortunate, but true. Thanks vogelheim@ for catching
this.

BUG=

Review URL: https://codereview.chromium.org/585023002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#295782}

SummerLW/Perf-Insight-Report,benschmaus/catapult,sahiljain/catapult,sahiljain/catapult,SummerLW/Perf-Insight-Report,benschmaus/catapult,catapult-project/catapult,sahiljain/catapult,benschmaus/catapult,benschmaus/catapult,SummerLW/Perf-Insight-Report,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult-csm,sahiljain/catapult,SummerLW/Perf-Insight-Report,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult,sahiljain/catapult,SummerLW/Perf-Insight-Report,catapult-project/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult-csm,SummerLW/Perf-Insight-Report,catapult-project/catapult