Fix flaky webkit_tests due to subdir in /data/tombstones

Tools/Scripts/webkitpy/layout_tests/port/android.py parses the output
of "ls -n /data/tombstones" to find tombstone files. But on at least
one device, there was a subdirectory /data/tombstones/mdm present.
The subdirectory caused parsing to fail as Android ls omits the size
column for directories, causing a "list index out of range error" when
the code attempts to access last_tombstone[6].

This patch tweaks the ls command so the directory doesn't show up.

See sample ls output (from the problematic device) below:

chrome-bot@chromeperf62:~$ /b/build/slave/WebKit_Android__Nexus4_/build/src/third_party/android_tools/sdk/platform-tools/adb -s 0049f0871e9bb70b shell ls -n /data/tombstones
drwx------ 1000     1000              2013-08-13 21:25 mdm
-rw------- 1000     1000       168124 2013-10-29 20:57 tombstone_00
-rw------- 1000     1000       143115 2013-10-29 22:20 tombstone_01
-rw------- 1000     1000       120823 2013-10-29 23:34 tombstone_02
-rw------- 1000     1000       124196 2013-10-29 23:55 tombstone_03
-rw------- 1000     1000       115205 2013-10-30 01:04 tombstone_04
-rw------- 1000     1000       121478 2013-10-30 06:34 tombstone_05
-rw------- 1000     1000       123452 2013-10-28 22:52 tombstone_06
-rw------- 1000     1000       166575 2013-10-28 23:29 tombstone_07
-rw------- 1000     1000       124192 2013-10-29 03:08 tombstone_08
-rw------- 1000     1000       122268 2013-10-29 07:07 tombstone_09

TBR=dpranke@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/50683005

git-svn-id: svn://svn.chromium.org/blink/trunk@160926 bbb929c8-8fbe-4397-9dbb-9b2b20218538