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-Commit-Position: refs/heads/master@{#295083}

android_webview/libwebviewchromium.gypi

@@ -43,7 +43,7 @@
         '-Wl,--fatal-warnings',
       ],
     }],
-    ['android_webview_build==1 and use_system_stlport==1', {
+    ['android_webview_build==1 and use_system_stlport==1 and use_system_icu==0', {
       # ICU requires RTTI, which is not present in the system's stlport, so
       # we have to include gabi++. We can't include it in icu.gyp because
       # link_settings cannot be used inside target_conditions. This will be

build/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.