GTTF: Fix passing sharding flags to the test launcher.
GTTF: Fix passing sharding flags to the test launcher.

This changes syntax from --flag value to --flag=value.

BUG=236893

Review URL: https://codereview.chromium.org/65683002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@233948 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build