media/gpu/v4l2: don't use C++ string for string constants

V4L2MemoryToString() returns a string constant, so change its type to
const char* to save some processing and make it more generic.

Bug: None
Test: Chromium builds and runs for Kukui.
Change-Id: I4cab66813e702493cbbce52a9956021dfdb4e818
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1939997
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719889}

media/gpu/v4l2/v4l2_device.cc

@@ -1470,7 +1470,7 @@ gfx::Size V4L2Device::AllocatedSizeFromV4L2Format(
 }
 
 // static
-std::string V4L2Device::V4L2MemoryToString(const v4l2_memory memory) {
+const char* V4L2Device::V4L2MemoryToString(const v4l2_memory memory) {
   switch (memory) {
     case V4L2_MEMORY_MMAP:
       return "V4L2_MEMORY_MMAP";

media/gpu/v4l2/v4l2_device.h

@@ -400,7 +400,7 @@ class MEDIA_GPU_EXPORT V4L2Device
   static int32_t H264LevelIdcToV4L2H264Level(uint8_t level_idc);
 
   // Converts v4l2_memory to a string.
-  static std::string V4L2MemoryToString(const v4l2_memory memory);
+  static const char* V4L2MemoryToString(const v4l2_memory memory);
 
   // Composes human readable string of v4l2_format.
   static std::string V4L2FormatToString(const struct v4l2_format& format);