Get rid of _profilesIdMap

Use linear search instead. This way we can postpone assigning profile uid until its header is received from the backend.

BUG=324769
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/100833004

git-svn-id: svn://svn.chromium.org/blink/trunk@163263 bbb929c8-8fbe-4397-9dbb-9b2b20218538