Devtools: refactor shell configuration logic.

Devtools: refactor shell configuration logic.

This patch splits the shell configuration logic into part that computes the
final list of options (shell_config) and the part that configures the shell
abstraction accordingly.

This becomes needed as information that affects a shell run (script
command-line arguments and inferred checkout paths), and we would like
to have another (configuration file).

This patch also includesthe logic that infers default paths in Chromium-like
checkouts in the common configuration logic, so that it now applies to
`mojo_test` as well as to `mojo_run`.

R=qsr@chromium.org
BUG=

Committed: https://chromium.googlesource.com/external/mojo/+/035f2cc50b8e2aacb3b3a7a8dc504bedfb46bb90

[ppi, 2015-07-31 16:50:40]

Patchset #2 (id:20001) has been deleted

[ppi, 2015-07-31 16:50:47]

Patchset #1 (id:1) has been deleted

[ppi, 2015-07-31 16:59:30]

Hi Ben,

ptal.

[ppi, 2015-08-03 11:20:11]

Hi Ben,

addressed your offline comments, ptal.

[qsr, 2015-08-03 11:24:55]

lgtm

[ppi, 2015-08-03 11:32:32]

Committed patchset #3 (id:80001) manually as
035f2cc50b8e2aacb3b3a7a8dc504bedfb46bb90 (presubmit successful).