chrome-sdk: source ~/.bashrc by default.
chrome-sdk: source ~/.bashrc by default.

To keep the user's shell environment as intact as possible, we source
the user's ~/.bashrc before applying our overrides.

This has the undesirable behavior of re-applying the same
transformations to variables such as PATH, but is not much different
from re-running 'bash' in an existing interactive shell.

Also fixes a bug where the environment wasn't being set up when calling
'cros chrome-sdk' with a command to execute.

BUG=None
TEST=Manual, unit tests.

Change-Id: I8e360c7aae8a51b91502dabadd99193e700518bf
Reviewed-on: https://gerrit.chromium.org/gerrit/46560
Commit-Queue: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Reviewed-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite