Correct the description of command line switch: disable-browser-plugin-compositing.

Correct the description to reflect the right status. The original patch which changed the
flag to "by-default enabled" is here:

https://chromiumcodereview.appspot.com/12547016/diff/16001/content/public/common/content_switches.cc

Review URL: https://codereview.chromium.org/111433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239773 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239773