sync: add default constructor to EntryKernel to avoid UMR of dirty_ bit.
sync: add default constructor to EntryKernel to avoid UMR of dirty_ bit.

Also add comments as requested by chron's post-submit code review.

BUG=43613
TEST=SyncableDirectoryTest

Review URL: http://codereview.chromium.org/2013011

git-svn-id: http://src.chromium.org/svn/trunk/src@46840 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 40e60699dfccdde87d2776d4cc960ce1f441d272
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser