Fixed uninitialized variable in r252635.

This issue should not affect behavior, except fix Valgrind errors.

CommandBufferHelper::flush_generation_ is used to detect a flush generation
change.  Initial value is not important.

BUG=343269

Review URL: https://codereview.chromium.org/172543008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252715 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252715