Enable optimize max for GN builds + update docs

Optimize max enables O2 for official builds.  Normally release builds
are O2 but the official build is Os, affecting performance.
The GYP file was previously updated to enable optimize max,
which enables ltcg and O2.

Documentation updated to show GN builds in docs/getting_started.md

BUG=libyuv:642
R=kjellander@chromium.org

Review URL: https://codereview.chromium.org/2386093003 .

BUILD.gn

@@ -98,9 +98,11 @@ static_library("libyuv") {
     deps += [ ":libyuv_msa" ]
   }
 
-  if (is_nacl) {
-    # Always enable optimization under NaCl to workaround crbug.com/538243 .
+  # Always enable optimization for Release and NaCl builds (to workaround
+  # crbug.com/538243).
+  if (!is_debug || is_nacl) {
     configs -= [ "//build/config/compiler:default_optimization" ]
+    # Enable optimize for speed (-O2) over size (-Os).
     configs += [ "//build/config/compiler:optimize_max" ]
   }
 }