android_webview: go back to using system ICU.

Partial revert of android_webview: stop using system ICU. (patchset #1 id:1 of https://codereview.chromium.org/541753002/)

Reason for revert:
We can't accept this size increase at present; revert until we've worked out what to do about it.

Original issue's description:
> 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
>
> Committed: https://crrev.com/48e43572c0bfef2b1434d7f330da86cc7770b592
> Cr-Commit-Position: refs/heads/master@{#293510}

BUG=409851

Review URL: https://codereview.chromium.org/570383002

Cr-Original-Commit-Position: refs/heads/master@{#295083}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 96256bfea5c496aa70b1913555b1e1c7f166c426

common.gypi

@@ -1807,6 +1807,7 @@
 
         # 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.