Bug 1481725: change minimum bitrate for VGA to 150kbps r=bwc

and adapt min bitrate below that.
Limits taken from the webrtc.org simulcast bitrate table.

media/webrtc/signaling/src/media-conduit/VideoConduit.cpp

@@ -1678,8 +1678,8 @@ static ResolutionAndBitrateLimits kResolutionAndBitrateLimits[] = {
   {MB_OF(1920, 1200), KBPS(1500), KBPS(2000), KBPS(10000)}, // >HD (3K, 4K, etc)
   {MB_OF(1280, 720), KBPS(1200), KBPS(1500), KBPS(5000)}, // HD ~1080-1200
   {MB_OF(800, 480), KBPS(600), KBPS(800), KBPS(2500)}, // HD ~720
-  {MB_OF(480, 270), KBPS(300), KBPS(500), KBPS(2000)},
-  {tl::Max<MB_OF(400, 240), MB_OF(352, 288)>::value, KBPS(200), KBPS(300), KBPS(1300)}, // VGA, WVGA
+  {MB_OF(480, 270), KBPS(150), KBPS(500), KBPS(2000)}, // WVGA
+  {tl::Max<MB_OF(400, 240), MB_OF(352, 288)>::value, KBPS(125), KBPS(300), KBPS(1300)}, // VGA
   {MB_OF(176, 144), KBPS(100), KBPS(150), KBPS(500)}, // WQVGA, CIF
   {0 , KBPS(40), KBPS(80), KBPS(250)} // QCIF and below
 };