DevTools: Prevent setting huge textContent in UI.

Huge texts reduce rendering performance drastically, nor they are more helpful than a truncated version.

BUG=347917
R=pfeldman, loislo

Review URL: https://codereview.chromium.org/186693002

git-svn-id: svn://svn.chromium.org/blink/trunk@168545 bbb929c8-8fbe-4397-9dbb-9b2b20218538