Memoize the result of Entry.toURL()

This patch makes Entry.toURL() remember the result in the first call and return the cached result in subsequent calls, and makes the properties const in order to make it sure that toURL() returns the same result.

As I ran in my environment, toURL() took ~10us. With this patch, it takes <1us now. See crbug.com/370908 for detail. 

BUG=370908
TEST=trybot passes
TBR=kinuko@chromium.org tzik@chromium.org
# TBRing since LGTM has gotten in crrev.com/280463004.

Review URL: https://codereview.chromium.org/272143003

git-svn-id: svn://svn.chromium.org/blink/trunk@173959 bbb929c8-8fbe-4397-9dbb-9b2b20218538