Add comment on StyleInvalidator trace events

Warn that StyleInvalidator methods are performance sensitive and
add notes about its implementation.

TBR=pdr,haraken
NOTRY=true
only comment additions. confirmed compile pass.
BUG=410701

Review URL: https://codereview.chromium.org/650343004

git-svn-id: svn://svn.chromium.org/blink/trunk@183961 bbb929c8-8fbe-4397-9dbb-9b2b20218538