android_webview: stop using system ICU.

To support unbundled updates we need to stop depending on system
libraries. Disable using the system version of ICU and instead link
Chromium's copy.

This increases binary size by ~7MB.

BUG=409851

Review URL: https://codereview.chromium.org/541753002

Cr-Original-Commit-Position: refs/heads/master@{#293510}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 48e43572c0bfef2b1434d7f330da86cc7770b592

common.gypi

@@ -1797,7 +1797,6 @@
 
         # When building as part of the Android system, use system libraries
         # where possible to reduce ROM size.
-        'use_system_icu%': '<(android_webview_build)',
         'use_system_stlport%': '<(android_webview_build)',
 
         # Copy it out one scope.