Define copy rule for content_shell.pak required for running webkit_unit_tests on android.

This is required for CL, https://codereview.chromium.org/456413002, to run webkit_unit_tests on android.
In subsequenct CL from blink side, https://codereview.chromium.org/542003002, we should extend the android.py to start sending this file to the
device under test.

R=jochen@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/544903003

Cr-Commit-Position: refs/heads/master@{#293500}

content/content_shell.gypi

@@ -452,6 +452,16 @@
             'browser/devtools/devtools_resources.gyp:devtools_resources',
           ],
         }],
+        ['OS=="android"', {
+          'copies': [
+            {
+              'destination': '<(PRODUCT_DIR)',
+              'files': [
+                '<(PRODUCT_DIR)/content_shell/assets/content_shell.pak'
+              ],
+            },
+          ],
+        }],
       ],
       'actions': [
         {