Don't depend on WEBKIT_IMPLEMENTATION guarded geometry conversion c'tors

Don't depend on WEBKIT_IMPLEMENTATION guarded geometry conversion c'tors

Since these files compile without WEBKIT_IMPLEMENTATION set the implicit
WebRect -> WebCore::IntRect and related conversion c'tors aren't available.
This defines a set of utility convert() functions and uses those to make
the conversions explicit. When we switch CC geometry types we can delete
these (if the cc API ends up using gfx:: types) or switch them to convert
to the cc types.

BUG=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=155887

[nduca, 2012-09-11 00:26:14]

lgtm and holds my bikeshedding tongue