Deflake fast/frames/seamless/*

These tests are periodically failing because a newline has been added to
the 'textContent' of a frame. This seems to be because of asynchronicity
in when the frame's 'load' event triggers (either the next line has been
parsed, or it hasn't).

This patch avoids the problem by trimming the string before comparing it
to the expected string.

BUG=316380

Review URL: https://codereview.chromium.org/91363003

git-svn-id: svn://svn.chromium.org/blink/trunk@163260 bbb929c8-8fbe-4397-9dbb-9b2b20218538