Fix Chrome for Android build after r234502

TabModel class can't have pure virtual functions with no implementation
in the private code.

Added a stub implementation.

TBR=dfalcantara@chromium.org
BUG=265806
NOTRY=true

Review URL: https://codereview.chromium.org/70403003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234535 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234535
[android_webview] Add instrumented_libraries to known_issues.

The instrumented_libraries library is not needed for the
android_webview build and should not be included in it because
of license incompatibility.

BUG=None
TEST=None
R=torne@chromium.org

Review URL: https://codereview.chromium.org/61313003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234536 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234536