Move the asset dir below <(PRODUCT_DIR), try 2.

Move the asset dir below <(PRODUCT_DIR), try 2.

Because: crrev.com/594603003 moved the V8 snapshot into an external
file in the asset dir. This makes the asset dir content dependent
on the platform and build mode (debug vs release). Hence, the dir
can no longer be shared between debug + release builds.
crbug.com/500113 contains more information.

The previous attempt (crrev.com/1184323002) was a bit *too* simple
and got reverted for "breaking Android". Which, indeed, it did,
since it ended up no longer putting the snapshot into the .apk,
which, well, breaks Android.

This time I:
- made sure no references to <(PRODUCT_DIR)/../assets remain in the
  .gyp files.
- Manually tested on an Android device.

@reviewers: I'm a bit surprised the trybots wouldn't catch something
  like this. Is there a particular set of bots I should trigger?

R=rmcilroy@chromium.org,dtrainor@chromium.org
BUG=500113

Review URL: https://codereview.chromium.org/1185163004

Cr-Commit-Position: refs/heads/master@{#335656}