android: Merge CreateOffscreenContext and CreateContext into one function.

android: Merge CreateOffscreenContext and CreateContext into one function.

These refactor these two functions into a single one to 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 content_unittests content_unittests_apk
$ export CHROMIUM_OUT_DIR=out_android
$ build/android/test_runner.py gtest -s content_unittests --debug -vvv

ContentShell tests:
$ ninja -C out_android/Debug content_shell_apk
$ ninja -C out_android/Debug content_shell_test_apk
$ build/android/adb_install_apk.py --apk=ContentShell.apk --debug
$ build/android/test_runner.py instrumentation --test-apk=ContentShellTest --isolate-file-path content/content_shell_test_apk.isolate --debug -vv

AndroidWebView tests:
$ ninja -C out_android/Debug android_webview_apk
$ ninja -C out_android/Debug android_webview_test_apk
$ 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 -vvv

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

Review URL: https://codereview.chromium.org/1061763002

Cr-Commit-Position: refs/heads/master@{#324033}