Ensure chrome://tracing shows IndexedDB as option

Poke the tracing system during context initialization so "IndexedDB"
is listed among the trace sources.

Also, give Shared Worker processes a friendly name.

BUG=315255
R=nduca

Review URL: https://codereview.chromium.org/103663003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239303 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239303