Cherry pick [android_webview] Introduce AwAssets to reference assets inside the apk.

Cherry pick of
https://src.chromium.org/viewvc/chrome?view=rev&revision=284238
and regenerate makefiles.

[android_webview] Introduce AwAssets to reference assets inside the apk.

This change introduces a utility class AwAssets, accessible by native,
which is able to retrieve references (fd + offset + size) of assets
inside the apk. This is to enable direct mmap of uncompressed assets.
This change does NOT introduce yet any change to the WebView apk itself.
At current state, no behavioral change is intended.

BUG=394502
Committed:
https://src.chromium.org/viewvc/chrome?view=rev&revision=284238

Bug: 16723226
Change-Id: I19c09a1e1f06b610be451dfc08f0b3e41b6cc72a
27 files changed