Fix Tab.getWebContents for downstream instrumentation tests
Tab.getWebContents() was using JNI to fetch the WebContents object. This
is undesirable for several reasons, including performance impact and
thread access consistency. Remove this and prefer redirecting the query
to ContentViewcore.getWebContents() instead.
This fix is a temporary workaround until all downstream test sites can
be patched.
BUG=398263,414445
TBR=tedchoc@chromium.org
NOTRY=true
Review URL: https://codereview.chromium.org/
571813003
Cr-Commit-Position: refs/heads/master@{#294918}