event/size: make methods take values, not pointers.

This makes the following program valid:

----
package main

import (
    "fmt"

    "golang.org/x/mobile/event/size"
)

func foo() size.Event { return size.Event{} }

func main() {
    fmt.Println(foo().Bounds())
}
----

Previously, you would get:
./main.go:12: cannot call pointer method on foo()
./main.go:12: cannot take the address of foo()

Change-Id: I2801d18a04d56d1c7496cb008531d078490ccf86
Reviewed-on: https://go-review.googlesource.com/18356
Reviewed-by: David Crawshaw <crawshaw@golang.org>

event/size/size.go

@@ -38,7 +38,7 @@ type Event struct {
 
 // Size returns the window's size in pixels, at the time this size event was
 // sent.
-func (e *Event) Size() image.Point {
+func (e Event) Size() image.Point {
 	return image.Point{e.WidthPx, e.HeightPx}
 }
 
@@ -47,7 +47,7 @@ func (e *Event) Size() image.Point {
 //
 // The top-left pixel is always (0, 0). The bottom-right pixel is given by the
 // width and height.
-func (e *Event) Bounds() image.Rectangle {
+func (e Event) Bounds() image.Rectangle {
 	return image.Rectangle{Max: image.Point{e.WidthPx, e.HeightPx}}
 }