DevTools: trim popover text displayed in object popover

A very long text in popover is rendered way too long. This patch trims
end of the string if its lenght exceeds 10000 chars.

BUG=419666
R=aandrey, vsevik

Review URL: https://codereview.chromium.org/615853005

git-svn-id: svn://svn.chromium.org/blink/trunk@183137 bbb929c8-8fbe-4397-9dbb-9b2b20218538