android_webview: stop using system expat.

To support unbundled updates we need to stop depending on system
libraries. Disable using the system version of libexpat and instead link
Chromium's copy, and stop removing the expat sources from the checkout.

This increases binary size by ~64KB.

Also remove an obsolete comment about using the system version of
sqlite; this was never implemented and now will not be.

BUG=409851

Review URL: https://codereview.chromium.org/534603002

Cr-Commit-Position: refs/heads/master@{#293115}

android_webview/tools/known_issues.py

@@ -53,7 +53,6 @@ KNOWN_INCOMPATIBLE = {
         # Code we don't want to build/include by accident from the main chromium
         # src/ directory.
         'third_party/ashmem/*.[ch]',
-        'third_party/expat/files/lib',
         'third_party/libjpeg/*.[ch]',
     ],
     'third_party/icu': [

build/common.gypi

@@ -1797,9 +1797,6 @@
 
         # When building as part of the Android system, use system libraries
         # where possible to reduce ROM size.
-        # TODO(steveblock): Investigate using the system version of sqlite.
-        'use_system_sqlite%': 0,  # '<(android_webview_build)',
-        'use_system_expat%': '<(android_webview_build)',
         'use_system_icu%': '<(android_webview_build)',
         'use_system_stlport%': '<(android_webview_build)',