Fix a crash of the Content Shell for Android when showing videos in a ContentVideoView.

The Content Shell for Android crashes when it attempts to show a ContentVideoView(see below logs for details).
It only happens in the child Shell that newly opened, not in the initial Shell.
Because a ContentViewClient is set only for the initial Shell, but not the others.

Steps to reproduces:
1. Go to http://www.google.com and search YouTube.
2. Click the link of YouTube in the result pages. Then YouTube page is opened in a new Shell.
3. Click a play button on a video clip.

java.lang.NullPointerException
        at org.chromium.content.browser.ContentVideoView.showContentVideoView(ContentVideoView.java:176)
        at org.chromium.content.browser.ContentVideoViewLegacy.showContentVideoView(ContentVideoViewLegacy.java:128)
        at org.chromium.content.browser.ContentVideoView.<init>(ContentVideoView.java:150)
        at org.chromium.content.browser.ContentVideoViewLegacy.<init>(ContentVideoViewLegacy.java:57)
        at org.chromium.content.browser.ContentVideoView.createContentVideoView(ContentVideoView.java:378)
        at org.chromium.base.SystemMessageHandler.nativeDoRunLoopOnce(Native Method)
        at org.chromium.base.SystemMessageHandler.handleMessage(SystemMessageHandler.java:24)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5017)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
        at dalvik.system.NativeStart.main(Native Method)

Review URL: https://codereview.chromium.org/172043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252530 0039d316-1c4b-4281-b951-d872f2087c98
Clean-up: Removes an unused forward declaration in canvas.h.

TEST=none
TBR=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/174753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252531 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252531