Make import-insert.html less flaky

The test used zero-timeout retry. This change replaces it
with exponential backoff.

TEST=import-insert.html
BUG=none
R=tkent,haraken

Review URL: https://codereview.chromium.org/99423009

git-svn-id: svn://svn.chromium.org/blink/trunk@163755 bbb929c8-8fbe-4397-9dbb-9b2b20218538