process: Implement platform independent MemoryPercent()

It calls mem.VirtualMemory() and process.MemoryInfo(), so on the platforms where
those work MemoryPercent() will as well.

Tested on Darwin.

process/process.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/shirou/gopsutil/cpu"
 	"github.com/shirou/gopsutil/internal/common"
+	"github.com/shirou/gopsutil/mem"
 )
 
 var invoke common.Invoker
@@ -145,3 +146,20 @@ func calculatePercent(t1, t2 *cpu.CPUTimesStat, delta float64, numcpu int) float
 	overall_percent := ((delta_proc / delta) * 100) * float64(numcpu)
 	return overall_percent
 }
+
+// MemoryPercent returns how many percent of the total RAM this process uses
+func (p *Process) MemoryPercent() (float32, error) {
+	machineMemory, err := mem.VirtualMemory()
+	if err != nil {
+		return 0, err
+	}
+	total := machineMemory.Total
+
+	processMemory, err := p.MemoryInfo()
+	if err != nil {
+		return 0, err
+	}
+	used := processMemory.RSS
+
+	return (100 * float32(used) / float32(total)), nil
+}

process/process_darwin.go

@@ -274,9 +274,6 @@ func (p *Process) MemoryInfo() (*MemoryInfoStat, error) {
 func (p *Process) MemoryInfoEx() (*MemoryInfoExStat, error) {
 	return nil, common.NotImplementedError
 }
-func (p *Process) MemoryPercent() (float32, error) {
-	return 0, common.NotImplementedError
-}
 
 func (p *Process) Children() ([]*Process, error) {
 	pids, err := common.CallPgrep(invoke, p.Pid)

process/process_freebsd.go

@@ -195,9 +195,6 @@ func (p *Process) MemoryInfo() (*MemoryInfoStat, error) {
 func (p *Process) MemoryInfoEx() (*MemoryInfoExStat, error) {
 	return nil, common.NotImplementedError
 }
-func (p *Process) MemoryPercent() (float32, error) {
-	return 0, common.NotImplementedError
-}
 
 func (p *Process) Children() ([]*Process, error) {
 	pids, err := common.CallPgrep(invoke, p.Pid)

process/process_linux.go

@@ -201,9 +201,6 @@ func (p *Process) MemoryInfoEx() (*MemoryInfoExStat, error) {
 	}
 	return memInfoEx, nil
 }
-func (p *Process) MemoryPercent() (float32, error) {
-	return 0, common.NotImplementedError
-}
 
 func (p *Process) Children() ([]*Process, error) {
 	pids, err := common.CallPgrep(invoke, p.Pid)

process/process_windows.go

@@ -227,9 +227,6 @@ func (p *Process) MemoryInfo() (*MemoryInfoStat, error) {
 func (p *Process) MemoryInfoEx() (*MemoryInfoExStat, error) {
 	return nil, common.NotImplementedError
 }
-func (p *Process) MemoryPercent() (float32, error) {
-	return 0, common.NotImplementedError
-}
 
 func (p *Process) Children() ([]*Process, error) {
 	return nil, common.NotImplementedError