Quota: Clamp the request quota size to the range of valid values

This adds [Clamp] attribute to requesPersistentQuota to avoid out of range
values according to the revised draft:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=24474


BUG=256288
TEST=run_webkit_tests.sh storage/quota\*

Review URL: https://codereview.chromium.org/150183002

git-svn-id: svn://svn.chromium.org/blink/trunk@166446 bbb929c8-8fbe-4397-9dbb-9b2b20218538