android_webview: Remove dependency on webkit's ContextProviderInProcess.

android_webview: Remove dependency on webkit's ContextProviderInProcess.

Instead build our own simplified version, so we don't need to depend on
webkit/common/gpu.

This should help ease the transition of
https://codereview.chromium.org/920443003.

Tested on Android (with Nexus 5 device) with the following command
lines:

$ ./build/gyp_chromium -DOS=android -Goutput_dir=out_android
$ ninja -C out_android/Debug android_webview_apk android_webview_test_apk
$ export CHROMIUM_OUT_DIR=out_android
$ build/android/adb_install_apk.py --apk=AndroidWebView.apk --debug
$ build/android/test_runner.py instrumentation --test-apk=AndroidWebViewTest \
  --test_data webview:android_webview/test/data/device_files --debug

BUG=338338
TEST=see above
R=boliu@chromium.org

Review URL: https://codereview.chromium.org/1083843002

Cr-Commit-Position: refs/heads/master@{#326968}