In case of cast_shell_pak built prior to cast_shell_icudata, the cast_shell.apk will be removed from assets directory when building cast_shell_icudata. We should not clear the assets directory when building cast_shell_icudata.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c058232e6063829ab7db5dab45bdbdbb988b06d0

commit c058232e6063829ab7db5dab45bdbdbb988b06d0
Author: tsunghung <tsunghung@chromium.org>
Date: Wed Mar 09 20:34:08 2016

Add the missing asset file cast_shell.pak back

All files under assets directory will be removed when building cast_shell_icudata target. We have to make sure all the asset files must be collected here.

BUG= 593277 

Review URL: https://codereview.chromium.org/1776933002

Cr-Commit-Position: refs/heads/master@{#380202}

[modify] https://crrev.com/c058232e6063829ab7db5dab45bdbdbb988b06d0/chromecast/chromecast.gyp

The change https://codereview.chromium.org/1776933002 breaks chromecast builds. Chromecast builds requires /assets/cast_shell.pak, but cast_shell_icudata target only applies to Android builds.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/89e5e9707432b52b19046f8aedee9021aae97e31

commit 89e5e9707432b52b19046f8aedee9021aae97e31
Author: alokp <alokp@chromium.org>
Date: Thu Mar 10 19:12:22 2016

Revert of Add the missing asset file cast_shell.pak back (patchset #3 id:40001 of https://codereview.chromium.org/1776933002/ )

Reason for revert:
Break chromecast gyp build:
"unknown target 'assets/cast_shell.pak'"

Original issue's description:
> Add the missing asset file cast_shell.pak back
>
> All files under assets directory will be removed when building cast_shell_icudata target. We have to make sure all the asset files must be collected here.
>
> BUG= 593277 
>
> Committed: https://crrev.com/c058232e6063829ab7db5dab45bdbdbb988b06d0
> Cr-Commit-Position: refs/heads/master@{#380202}

TBR=michaelbai@chromium.org,halliwell@chromium.org,sanfin@chromium.org,tsunghung@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG= 593277 

Review URL: https://codereview.chromium.org/1782133003

Cr-Commit-Position: refs/heads/master@{#380440}

[modify] https://crrev.com/89e5e9707432b52b19046f8aedee9021aae97e31/chromecast/chromecast.gyp

Gyp is deprecated, will work on gn build only.