Fixes possible crash in tab dragging

Fixes possible crash in tab dragging

Crash would occur in resetting selection model. It was possible to
attempt to reset the selection model with an active index of -1, which
TabStripModel does not like.

BUG=338285
TEST=see bug
R=ben@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=247868

[sky, 2014-01-29 23:02:52]

Test will have to wait until I refactor for better testing.

[Ben Goodger (Google), 2014-01-29 23:16:30]

lgtm

[commit-bot: I haz the power, 2014-01-29 23:43:40]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sky@chromium.org/149393009/1

[commit-bot: I haz the power, 2014-01-30 00:34:26]

Retried try job too often on win_rel for step(s) app_list_unittests,
ash_unittests, aura_unittests, cacheinvalidation_unittests, cc_unittests,
check_deps, check_deps2git, chrome_elf_unittests, chromedriver_unittests,
components_unittests, compositor_unittests, content_browsertests,
content_unittests, crypto_unittests, events_unittests, google_apis_unittests,
gpu_unittests, installer_util_unittests, ipc_tests, jingle_unittests,
media_unittests, mini_installer_test, nacl_integration, ppapi_unittests,
printing_unittests, remoting_unittests, sql_unittests, sync_integration_tests,
sync_unit_tests, telemetry_perf_unittests, telemetry_unittests, views_unittests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[commit-bot: I haz the power, 2014-01-30 04:25:40]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sky@chromium.org/149393009/1

[commit-bot: I haz the power, 2014-01-30 07:18:42]

Change committed as 247868