weblayer: Use mobile fling curve on android

Do this by changing the default implementation to true for OS_ANDROID.
This also allows removing the android webview override.

Bug: 1038894
Change-Id: I1be3c680a32116bc8867958d85f89ad5302a525d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006208
Commit-Queue: Bo <boliu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732842}

android_webview/browser/aw_content_browser_client.cc

@@ -405,10 +405,6 @@ bool AwContentBrowserClient::IsExplicitNavigation(
   return ui::PageTransitionCoreTypeIs(transition, ui::PAGE_TRANSITION_TYPED);
 }
 
-bool AwContentBrowserClient::ShouldUseMobileFlingCurve() {
-  return true;
-}
-
 bool AwContentBrowserClient::IsHandledURL(const GURL& url) {
   if (!url.is_valid()) {
     // We handle error cases.

android_webview/browser/aw_content_browser_client.h

@@ -71,7 +71,6 @@ class AwContentBrowserClient : public content::ContentBrowserClient {
       content::WebContents* web_contents) override;
   void RenderProcessWillLaunch(content::RenderProcessHost* host) override;
   bool IsExplicitNavigation(ui::PageTransition transition) override;
-  bool ShouldUseMobileFlingCurve() override;
   bool IsHandledURL(const GURL& url) override;
   bool ForceSniffingFileUrlsForHtml() override;
   void AppendExtraCommandLineSwitches(base::CommandLine* command_line,

content/public/android/javatests/src/org/chromium/content/browser/ContentViewScrollingTest.java

@@ -233,7 +233,7 @@ public class ContentViewScrollingTest {
         int velocity = (int) (1000 * deviceScaleFactor);
         // Expected total fling distance calculated by FlingCurve with initial
         // velocity 1000.
-        int expected_dist = (int) (180 * deviceScaleFactor);
+        int expected_dist = (int) (194 * deviceScaleFactor);
 
         // Vertical fling to lower-left.
         fling(0, -velocity);

content/public/browser/content_browser_client.cc

@@ -102,7 +102,11 @@ bool ContentBrowserClient::IsExplicitNavigation(ui::PageTransition transition) {
 }
 
 bool ContentBrowserClient::ShouldUseMobileFlingCurve() {
+#if defined(OS_ANDROID)
+  return true;
+#else
   return false;
+#endif
 }
 
 bool ContentBrowserClient::ShouldUseProcessPerSite(