Add ability to specify gyp dir to gyp_to_android.

Add ability to specify gyp dir to gyp_to_android.

When running gyp_to_android inside a Skia tree, it uses the gyp program
in third_party/externals. Inside an Android tree, we need to use a
different location for gyp (the Android copy of Skia, in external/skia,
does not contain third_party/externals).

The bot handles this by importing gyp_to_android and running its main
function directly.

Add command line arguments so a human can directly run gyp_to_android.

Committed: https://skia.googlesource.com/skia/+/8fd8d38626ebb84d47ed8ba155efecb241fed6dc

[scroggo, 2015-01-20 16:40:00]

scroggo@google.com changed reviewers:
+ borenet@google.com

[borenet, 2015-01-20 18:16:31]

LGTM

[scroggo, 2015-01-20 18:27:06]

The CQ bit was checked by scroggo@google.com

[commit-bot: I haz the power, 2015-01-20 18:27:51]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/864503002/1

[commit-bot: I haz the power, 2015-01-20 18:39:28]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/8fd8d38626ebb84d47ed8ba155efecb241fed6dc