Rewrite how skydb starts prompt.cc

Rewrite how skydb starts prompt.cc

This makes skydb pass a command_port to prompt.cc
as a commandline argument.  I removed passing of the
url and instead pass the url via a separate /load command.

This has the nice side effect of guarenteeing that
the sky instance is up and ready to respond to commands
when skydb exits.  It also prepares us for running
more than one copy of prompt.cc as jamesr requested
for both Release and Debug or for on both your android
device as well as your local machine.

R=abarth@chromium.org, jamesr@chromium.org
BUG=

Committed: https://chromium.googlesource.com/external/mojo/+/9958496d43ac5d002dabfb453a0e19bc540b624d

[abarth-chromium, 2015-01-08 23:49:36]

LGTM

[eseidel, 2015-01-08 23:50:14]

Committed patchset #2 (id:20001) manually as
9958496d43ac5d002dabfb453a0e19bc540b624d (presubmit successful).

[jamesr, 2015-01-08 23:50:38]

lgtm

https://codereview.chromium.org/841113003/diff/20001/sky/tools/debugger/promp...
File sky/tools/debugger/prompt/prompt.cc (right):

https://codereview.chromium.org/841113003/diff/20001/sky/tools/debugger/promp...
sky/tools/debugger/prompt/prompt.cc:159: unsigned command_port_;
please use uint32_t.  unsigned is longer to type and ambiguous about how many
bits it holds