Android: Turn on png to webp for chrome

This CL enables webp conversion for ChromeModern and Monochrome.
SystemWebview will be enabled in: http://crrev.com/c/565542

ChromeModern, Monochrome, and SystemWebview apks all have min sdk
version greater than 18. Thus webp is fully supported on those versions
and since they are actually all API 21+, optimizations in L, M, and N
will be applicable to all devices that receive those apks.

This is enabled for all release builds so that if any issues crop up
just a simple release build will trigger the conversion. On dev machines
it is expected to add about 30 seconds to the overall build time, up to
1 minute if the machine is under load.

BUG=695482

Change-Id: Ic4377b9642e09eb1c886626f7750ad00951746e7
Reviewed-on: https://chromium-review.googlesource.com/561578
Commit-Queue: Peter Wen <wnwen@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485321}

chrome/android/BUILD.gn

@@ -861,6 +861,10 @@ chrome_public_apk_tmpl_shared("chrome_modern_public_apk") {
   apk_name = "ChromeModernPublic"
   shared_libraries = [ ":chrome" ]
 
+  if (!is_java_debug) {
+    png_to_webp = true
+  }
+
   # Always enable load_library_from_apk.
   load_library_from_apk = chromium_linker_supported
 }

chrome/android/chrome_public_apk_tmpl.gni

@@ -143,6 +143,7 @@ template("monochrome_public_apk_tmpl") {
         proguard_configs = []
       }
       proguard_configs += [ "//android_webview/apk/java/proguard.flags" ]
+      png_to_webp = true
     }
   }
 }