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The Page class was being declared in page/page.py which was creating problems
for several module-level functions handling pages which were being declared
(CL to follow). This CL kills page.py and moves the associated logic into
page/__init__.py, in line with e.g. value/.

R=dtu,nednguyen,chrishenry

Review URL: https://codereview.chromium.org/556453003

Cr-Commit-Position: refs/heads/master@{#296899}