Try to deflake editing/spelling/spelling-huge-text.html.

Give the test more time to complete. It's timing out on Linux (dbg).

BUG=298047
TBR=g.czajkowski@samsung.com

Review URL: https://codereview.chromium.org/24497007

git-svn-id: svn://svn.chromium.org/blink/trunk@158381 bbb929c8-8fbe-4397-9dbb-9b2b20218538