Before this CL, chrome://memory-internals stops listing up
processes, if a "Tab" process does not have histories.
This CL fix it with following changes.

1) Check if the index 'history' exists before accessing it.
2) Changed the order of updating, from
> process table -> extension table -> OS and uptime -> JSON
to
> JSON -> OS and uptime -> process table -> extension table,
because the JSON string is the original data, and tables
may contain unexpected bugs.  If the JSON is shown, it is
easy to find why an update stops working.



BUG=338451,309913
TEST=Open chrome://memory-internals
NOTRY=true

Review URL: https://codereview.chromium.org/148233005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247475 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247475