Refactor/cleanup WebRTC-specific Andorid glue code.

Refactor/cleanup WebRTC-specific Andorid glue code.

Remove a bunch of code that is already longer used or
will become unused when https://review.webrtc.org/22149004/
and https://codereview.chromium.org/505153002/ are landed.

Remove hardcoded paths to the WebRTC isolate files to make
it easier for us to make changes to them, since we can pass
the path to the .isolate file on test execution instead
(keeping the configuration in the WebRTC buildbot recipe instead).
Previously we had to roll our Chromium revision in WebRTC DEPS
every time a change was made before it became used.

Add support for a CHECKOUT_SOURCE_ROOT environment variable
used to make it possible to override the hardcoded path traversal
that is done to find the src/ directory (since it's different for
WebRTC bots). I considered passing a flag into the test_runner.py
scripts instead of using an environment variable, but my opinion is
that it would pollute the code too much since
constants.DIR_SOURCE_ROOT is used in 70 different places.

BUG=webrtc:3741
TEST=local building and test execution using command lines like this:
cd /path/to/webrtc/src
export CHECKOUT_SOURCE_ROOT=`pwd`
build/android/test_runner.py gtest -s tools_unittests --isolate-file-path=webrtc/tools/tools_unittests.isolate

R=navabi@chromium.org

Review URL: https://codereview.chromium.org/500423004

Cr-Commit-Position: refs/heads/master@{#292861}