[android_webview] Use a fraction to calculate scroll offset.

[android_webview] Use a fraction to calculate scroll offset.

android_webview/ has to expose scroll offset operations in physical
pixels while cc/ stores scroll offsets in dip pixels.
Unfortunately the physical pixel values android_webview/ exposes are
rounded so there is no such combination of rounding operations that
allows for scrolls to 'bottom' in one corrdinate space to always be
'flush' against the bottom edge in the other. The most natural
solution is to convert the scroll offset to the fraction of the
scroll range.
By using the max_scroll_offset value obtained directly from the compositor
we're also removing the need to delay scrolls since the content size
is always as recent as the scroll offset update.

BUG=261239

Review URL: https://codereview.chromium.org/23533051

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225348 0039d316-1c4b-4281-b951-d872f2087c98