[Android WebView] Fix initial scale override application for non-wide viewport

A follow-up for https://codereview.chromium.org/25353006. The CL has caused
AwSettingsTest.testSetInitialScale to start failing.

In non-wide viewport mode, after the page scale has been being reset,
it is possible that the initial scale override will be outside of
the newly set up min-max range, and thus will be clamped.

As we anyway take into account m_userAgentConstraints.initialScale in the
non-wide viewport mode case, we should also use it for resetting the initial
scale.  This follows the legacy behavior -- for user-zoomable pages, the
target density dpi provided will only affect the layout width, but not scale.

BUG=303189, 304548

Review URL: https://codereview.chromium.org/70243003

git-svn-id: svn://svn.chromium.org/blink/trunk@161853 bbb929c8-8fbe-4397-9dbb-9b2b20218538