Move the asset dir below <(PRODUCT_DIR).
Move the asset dir below <(PRODUCT_DIR).

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.

@reviewers: I don't really understand the Android build + packaging,
so I'm not at all sure if this has any unwanted side effects. Feel free
to reject the CL if it may cause issues elsewhere.

BUG=500113

Review URL: https://codereview.chromium.org/1184323002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#334788}

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