Added back initialization of m_scale in PinchViewport

Fixes uninitialized usage valgrind warnings.

BUG=372226

Review URL: https://codereview.chromium.org/280473002

git-svn-id: svn://svn.chromium.org/blink/trunk@173871 bbb929c8-8fbe-4397-9dbb-9b2b20218538