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* Browser has new properties supports_extensions and extensions (of type ExtensionDict)
* BrowserBackend ctor takes additional arg supports_extensions. 
* BrowserBackend has new properties supports_extensions and extension_dict_backend (of type ExtensionDictBackend)
* BrowserBackend._WaitForBrowserToComeUp waits for all extensions to load and be interactive.
* BrowserBackend.ExtensionsNotSupportedException is raised for android browser and content shell.
* Add class ExtensionToLoad which is a wrapper for an extension_path.
* Add extension_to_load (of type ExtensionToLoad) to BrowserOptions.
* Add SupportsOptions to PossibleBrowser, to test extensions support. Returns False for android and content_shell when BrowserOptions.extensions_to_load is not empty 
* Add ExtensionPage which derives from WebContents
* Add ExtensionDictBackend which is a dictionary of ExtensionPage instances with extension ids as keys.
* Add ExtensionDict which passes through to ExtensionDictBackend, except it uses ExtensionToLoad instances as keys.
* Import crx_id for determining extension id from extension path.
* Simple extension manifest.json and background.js for unit tests.
* Unit tests for single and multiple extensions.

BUG=169954
TEST=browser test.
Review URL: https://codereview.chromium.org/11882033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@182316 0039d316-1c4b-4281-b951-d872f2087c98