Roll libvpx 55f7ba:efe971

Ensure we are not building any of the arm code with gcc4.6
https://codereview.chromium.org/597033002

R=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/597703004

Cr-Commit-Position: refs/heads/master@{#296400}

DEPS

@@ -33,7 +33,7 @@ vars = {
   'pdfium_git': 'https://pdfium.googlesource.com',
   'skia_git': 'https://skia.googlesource.com',
   'boringssl_git': 'https://boringssl.googlesource.com',
-  'libvpx_revision': '55f7ba2f1239ae021542c0524b777010a1d885b6',
+  'libvpx_revision': 'efe9712d52c2d216fb3d1ceb508b8148847a7e4b',
   'sfntly_revision': '1bdaae8fc788a5ac8936d68bf24f37d977a13dac',
   'skia_revision': 'cd523ebe2be30f268cdcec2d0b59eba609b9901b',
   # Three lines of non-changing comments so that