[Android] Restore EGL Context after collecting GPU info

In Android, a short lived EGL context is created on the UI thread during
start up to collect GPU info. One side effect is that if there is an
existing EGL context current, then it is released. This is a problem for
Android WebView but may affect Chrome as well.

This patch saves and restores the existing current context if there is
one.

BUG=

Review URL: https://codereview.chromium.org/25298003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226104 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226104