SerializedScriptValue's Writer calls realloc a lot

Currently SerializedScriptValue's Writer is resizing its StringBuffer to the
exact size it needs for every object it writes into the buffer. Instead, it
should grow the buffer exponentially to avoid calling realloc so many times.

This CL fixes a regression from Blink@155225. I've also removed a call to
isolatedCopy which is unnecessary.

R=jsbell@chromium.org
BUG=266837

Review URL: https://chromiumcodereview.appspot.com/24043003

git-svn-id: svn://svn.chromium.org/blink/trunk@157388 bbb929c8-8fbe-4397-9dbb-9b2b20218538