[chromedriver] Accept and ignore "loadAsync" for android.

BUG=None

Review URL: https://codereview.chromium.org/123833007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243407 0039d316-1c4b-4281-b951-d872f2087c98
[chromedriver] Reuse forwarded adb ports.

adb forward --remove is not available in older adb binaries;
to avoid leaks, maintain a pool of previously forwarded ports.

BUG=None

Review URL: https://codereview.chromium.org/127143003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243830 0039d316-1c4b-4281-b951-d872f2087c98