Fix to remove customised String over IPC for SmartClip

Currently, the data for SmartClip is sent over IPC as a customized
string, which contains rect values and string of content within the
clip rect. Changes from Blink side to remove custom serialization and
send it as a structure of rect and string values.

BUG=330872

Review URL: https://codereview.chromium.org/259263003

git-svn-id: svn://svn.chromium.org/blink/trunk@173295 bbb929c8-8fbe-4397-9dbb-9b2b20218538