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Set CACHE_DIR to WV-specific directory (WebView).
Folders under app_webview are moved into app_webview/Default.

HTTP Cache under cache/org.chromium.android_webview is moved into
cache/WebView/Default/HTTP Cache.

Bug: 963875
Change-Id: Ifa150fe0a40a775bc1705e1ac4ea355e67dbee0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1793087
Commit-Queue: Anna Malova <amalova@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702833}