Lots of places in the code currently call the two-arg version of
ConvertJavaStringTo* with a JNI method parameter, which is now relying
on the implicit conversion from JavaParamRef<jstring> to jstring. To
avoid this, define a two-arg version that takes a JavaRef as the second
parameter. If we have the environment it's more efficient to keep it
around rather than change all the callers to the one-arg version, since
the one-arg version has to call AttachCurrentThread.

BUG=506850
R=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/1321423005

Cr-Commit-Position: refs/heads/master@{#348874}