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: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@46840 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser