1. Removed |net_small| and |url_small| targets.
2. To keep supporting the small builds, introduced |use_platform_icu_alternatives|
   boolean flag that indicates whether the net and url targets should be
   built with ICU alternatives. The default value of the flag is false.
3. Removed |cronet_static_small| target.
4. Renamed 'define' USE_ICU_ALTERNATIVES_ON_ANDROID to
   USE_PLATFORM_ICU_ALTERNATIVES.
5. Added use_platform_icu_alternatives=1/true to gyp/gn cr_cronet.py.
6. Changed cronet_reduction_proxy component to stop using net & url small.
7. Removed |common_small| template from data_reduction_proxy/core/common
8. Removed |google_apis_small| template from google_apis.
9. Added simplistic ICU alternatives for iOS
10. Fixed linking problem when net_unitests were build with GN (not all
    websockets tests were excluded when enable_websockets was set to false).
11. Added support for net_unitests for the small net/url variation
    (there were no tests for it before).
12. Created the list of unit tests that are failing with the current
    alternative ICU implementations on Android and iOS. Added them to the
    exclude list.
13. Added new flag 'disable_brotli_filter' which is false by default.
    Set the flag to true in cr_cronet.py.
14. Converted 'define' USE_PLATFORM_ICU_ALTERNATIVES to build flags.

The current number of passing tests:
net_with_icu_alternative iOS: 12548
net_with_icu iOS: 14549
net_with_icu_alternative Android: 15000
net_with_icu Android: 15223
url_with_icu_alternative iOS: 49
url_with_icu iOS: 91
url_with_icu_alternative Android: 50
url_with_icu Android: 92

BUG=601172

Review URL: https://codereview.chromium.org/1839803002

Cr-Commit-Position: refs/heads/master@{#387382}